Preheat oven (electric cooker: 175 °C/ circulating air: 150 °C/ gas: see manufacturer) Grease square springform pan (24 x 24 cm) and dust with flour. Mix flour, 25 g cocoa and baking powder. Beat the eggs, 1 pinch of salt and brown sugar with the whisk of the mixer until thick and creamy.
Stir in oil and buttermilk. Sift the flour mixture on top and stir in briefly. Fill the dough into the prepared form and smooth it down. Bake in a hot oven for about 12 minutes. Remove and let cool on a cake rack.
For the chocolate toffee, break the dark chocolate into large pieces. Heat 150 g sugar and 5 tbsp water in a pan and caramelise until golden brown. Add 150 g cream and bring to the boil. Add chocolate and simmer while stirring until caramel and chocolate have melted.
Remove from the stove and let it cool lukewarm.
Remove the cake from the mould. Place the ring around the base again. Roast the hazelnuts in a pan without fat. Take out and let it cool down.
Beat the egg whites over a warm water bath with the whisks of the mixer until stiff. Add 100 g sugar and 1 pinch of salt. Beat until the mixture is shiny and the sugar has dissolved.
Spread meringue cream on the floor. Spread caramel cream on top. Sprinkle with hazelnuts. Whip 250 g cream until stiff. Spread carefully on the chocolate toffee and chill for about 30 minutes.
Remove from the mould and dust with 1 tablespoon of cocoa. Cut into pieces and decorate with strawberries as desired.