Olkiluoto3torttu

The third unit of the Olkiluoto power plant has finally started regular operations! And how to celebrate the olkiluoto päivä without an olkiluoto3torttu? So, here's the recipe:

The structure of the cake is inspired by the look of the power plant: alternating brown and white mousse layers sandwiched between black base and topping:

Ingredients (assuming ~30x30x5cm final size):

39 digestive biscuits (assuming 15g/biscuit, I used Xtra Digestive täysjyvävehnäkeksi)
~185g of black tahini
~1kg of hazelnuts or 800g of roasted hazelnut paste/butter (some of the weight will get lost during roasting and processing)
2 white chocolates (2*128g - I used a “normal” Panda valko sulkaa, not the baking variety)
200ml of 3.5% milk
1410ml of cream (I used kuohukerma, which is 35%)
195g of sugar
gelatin (liivatejauhe)

Equipment:

A fridge and a freezer. Enough free space in the freezer to remove one of the drawers
An oven and a baking tray
A square, adjustable baking frame - I used the 30x30cm TÅRTBAK from IKEA
Baking paper
Some thick plastic foil/sheets: they are used to separate the baking frame from the cake. Without it opening the frame will likely damage the cake. Also the cake ended up being a bit taller than the frame, so without it you need to scale the amounts of ingredients down by ~10%. I've cut 4 pieces from stiff document sleeves (I have a lot of spare ones in case you want to try the recipe).
A chopper (for making the hazelnut paste and crushing cookies)
Some tool to make a whipped cream with
A large bowl (having more than one will be useful)
An electronic kitchen scale and volume measuring equipment

The process: The cake is build layer by layer. Only the base requires baking, the rest is put on top the previous layers and put into freezer to solidify. Solidifying in the fridge may not work for the white layers, I didn't test it.

The base:
1. Put 32 (so not all 39) biscuits into the chopper, add 154g of tahini and mix/chop well. Some tahini will likely stick to the walls of the chopper - use a spoon to mix it with the biscuits.
2. Put a baking sheet on the baking tray, and put the baking frame on top. Depending on the shape of the tray you make not be able to use the maximum size of the frame, but it should be close enough.
3. Put the tahini-biscuit mix into the frame and spread evenly.
4. Bake in 150°C for 10-15 minutes. This is just to solidify the mix.
5. Allow it to cool down outside of the oven
6. Install the plastic sheets - we do it now, because they would be destroyed in the oven
7. Put the whole thing into the freezer (with the baking tray). Just remove one of the drawers and put the baking tray in
The topping:
1. Put the remaining 7 biscuits and 30g of tahini into the chopper, mix/chop as previously
2. Keep the mix in the fridge for later use
3. Clean the chopper - you will need it later for the hazelnuts
The hazelnut butter/paste:
1. Roast the hazelnuts: you can use temperature between 150-175°C, the higher the quicker is the process, but also the easier it is to overdo the roasting. The peels should crack and the nuts should get light brown/orangey in color. To monitor the process, check what do they look like on the inside and taste them.
2. Remove the peels from the nuts (you don't need to be very thorough, mixing the nuts in a towel is enough). Let them cool a little bit and put them in the chopper. After some minutes of chopping you should produce hazelnut butter. You probably want to do it in 2 or 3 batches, depending on the size of the chopper.
The brown layers (x3):
1. each layer calls for: 310ml of cream, 266g of the hazelnut paste, 65g of sugar and a 1.25 teaspoon of gelatine powder
2. separate 30ml of the cream, and use the remaining 280ml to make a whipped cream. Store the whipped cream in the fridge.
3. add the 65g of sugar to the 30ml of cream and put on low heat/water bath (I used a metal mug, which I put on an frying pan with some water to buffer the heat and avoid burning the cream). Mix to dissolve most of the sugar.
4. Add the cream-sugar mix to the 266g of hazelnut paste, mix well.
5. Prepare gelatin: combine it with 3 table spoons (the bigger ones, not tea spoons) of water and put on a water bath. Once it soaks the water it should start dissolving - you can help by mixing it a bit.
6. Add gelatin to the hazelnut paste an mix very well.
7. Add 1/4 of the whipped cream to the hazelnut paste and mix gently
8. Combine the paste with the rest of the cream
9. Remove the baking tray from the freezer, put the mix into the baking frame and spread somewhat evenly - it won't be perfect, don't worry too much
10. Put the tray back into the freezer and give it some time to solidify (~30 minutes)
11. Start working on the next layer
The white layers (x2):
1. each layer calls for: 100ml of milk, 240ml of cream, 128g white chocolate, 1 teaspoon of gelatin
2. whip the cream (all of it)
3. melt chocolate in the milk using microwave of a water bath, mix well
4. prepare gelatin like before, mix with the chocolate/milk mix
5. let it cool a bit and mix it gently with the whipped cream - the mix will be more of a liquid than the hazelnut, don't worry
6. Remove the baking tray from the freezer, pour the mix, help it to spread in the corners
7. Put the tray back into the freezer and give it some time to solidify (~30 minutes)
8. Start working on the next layer
Putting the topping:
1. Sprinkle the biscuit-tahini mix, you can use a butter knife to spread it around gently. Alternatively, you can first bake the mix in 150°C, crush it again and then sprinkle - it should be a bit easier, as the mix won't be that wet.

I didn't take many pictures during the baking but here are some:

the base before baking:

hazelnuts getting turned into paste:

The whole process of making the layers is a bit time-consuming, but the results are very good. The white layer turned out almost too runny, but was fine in the end. The cake contains relatively little simple sugar (~12%, including sugar from the chocolate and the biscuits), so that's a plus. There was enough cake to last for the whole open day.