Preheat oven to 180°C. Line a baking tray with parchment paper.
In a bowl, sift flour, potato starch, baking powder, and cocoa. Add sugar and whisk.
Add oil and chocolate milk. Mix until smooth.
Spread batter evenly in the tray. Bake 6–8 minutes (until set).
Cool completely, then cut the sponge into two equal halves.
Melt white chocolate over a double boiler and let it cool slightly.
Whip cold heavy cream with mascarpone until thick and smooth. Add cooled white chocolate and mix until combined.
Spread filling over one sponge half. Cover with the second half.
Chill until firm, then slice. Decorate with melted chocolate or Nutella if you like.