Preheat the oven to 175°C. Prepare the cake tin by greasing the sides and attaching the baking paper. You can use one regular-sized cake tin or two smaller ones (I was using two smaller tins).
Cook the pistachio pudding by following the instructions on the packaging. Let the pudding completely cool.
Start by creaming egg yolks, sugars, and coffee powder. Mix for 4-5 minutes until creamy.
Add milk, oil, sour cream, and vanilla extract. Mix well.
Continue by adding dry ingredients - flour, baking powder, cocoa powder. Mix again and make sure you scrape down the bowl. The consistency can be very paste-like.
Whip the egg whites. Fold them in very gently with a rubber spatula. Be careful.
Pour the cake mix into the cake tins. Add pistachio pudding - one tablespoon at a time, to different places on top of the cake mix.
Bake for 32-40 minutes or until the toothpick comes out clean. Take the cakes out of the oven and let cool completely.
In the meantime whip the whipping cream with cream stiffener and icing sugar. Put into the fridge for at least 10 minutes.
When the cake is completely cool, put the cream on top with fresh strawberries. Sprinkle some pistachio on top as well. I recommend serving a piece of cake after 2 hours in the fridge.