Preheat the oven for the dough (circulating air: 180 °C). Grease two springforms (25 cm Ø each) with some butter. Mix 280 g flour, starch, baking soda, baking powder, 300 g sugar and salt in a large bowl. Mix eggs, buttermilk and vanilla extract. Cut 200 g butter into the dry ingredients. Work in roughly with the whisk of the mixer or with a food processor for about 30 seconds.
Add the buttermilk and egg mixture in 3 portions, then stir for 15 seconds each time until everything is well mixed. Spread evenly on the baking tins. Bake the 3. doughs one after the other for approx. 25 minutes (stick test!). Cool for about 10 minutes, then remove from the pan and allow to cool.
For the caramel sauce, caramelise 200 g sugar in a wide pot until golden brown. Stir in 90 g butter with a whisk. As soon as the butter has melted, remove the pot from the heat, then slowly add the cream. Let the caramel sauce cool down. The sauce will keep in the refrigerator for several weeks.
For the chocolate butter cream, mix 300 g butter in a bowl with the whisks of the mixer until creamy. Add icing sugar and continue beating. Stir in liquid chocolate well with a rubber spatula.
Cut the bottoms in half horizontally. Spread approx. 5 mm buttercream on one base. Pour 2-4 tablespoons caramel sauce over it. Place 2nd cake layer on top, spread 5 mm buttercream on top and pour caramel sauce over it. Place the base on top. Repeat the process. Place the base on top. Spread the rest of the buttercream on the cake. Decorate with small candles as desired.