Preheat the oven to thermostat 6 (180°C).
Whisk the eggs and sugar together in a heatproof bowl over a simmering double boiler for 1 minute. Remove from heat and place the bowl in ice water, beating until cold. Sift in the flour, baking powder, and Guanaja cocoa powder. Gently fold in without overmixing.
Butter and flour a springform tin, pour in the batter, and bake for 25-30 minutes.
For the buttercream: Boil sugar with 15 cl water for 5 minutes. Whip egg yolks with a mixer, add cocoa, then slowly pour in the hot syrup while whisking until cold. Gradually incorporate soft butter pieces, whisking to a smooth cream.
Remove cake from oven, unmold, and cool completely. Trim crust from edges. Slice horizontally in half. Spread one disc with buttercream, top with raspberries, add another cream layer. Cover with second disc. Dust with cocoa powder, arrange raspberry crown around edges and center, sprinkle chocolate shavings, and finish with icing sugar.
