Such an airy chocolate sponge roll

Easy fast and very chocolate sponge roll!
For cooking, take:
flour 80g
baking powder 1 tsp
cocoa 20g
eggs 4 pieces
a pinch of salt
sugar 120-150g
chocolate 50g
cook for 10 minutes at 180 ºС.
For the cream:
butter 100g
boiled condensed milk 100g
chocolate 100g.
For decoration:
200g white chocolate
50g milk chocolate
nuts