Step 1
Preheat the oven to 350 F.
Step 2
Whisk together the all-purpose flour, cocoa powder, baking powder, baking soda, and the sea salt.
Step 3
Beat the butter and sugar on a high setting for 2 to 3 minutes until light and creamy.
Step 4
Mix in the eggs and the vanilla extract.
Step 3
Fold in half of the flour and cocoa powder mixture.
Step 5
Fold in the remaining flour and cocoa powder.
Step 7
Transfer the cake batter to a lined 13×9-inch baking sheet and spread out evenly
Step 7
Bake for 40 to 45 minutes until a skewer inserted into the sponge comes out clean.
Step 9
Remove the cake from the oven and cool completely, for at least one hour.
Step 8
Once cooled remove the cake from the tin.
Step 10
Use a wire cutter or bread knife to slice the sponge in half.
Step 12
Add 2 tablespoons butter to the bowl containing the white chocolate, and 1 tablespoon butter each to the bowls containing milk and dark chocolate.
Step 12
Using the base of a plastic soda bottle, take turns pouring a little of each type of chocolate onto the bottle allowing it to flow out over the top of the sponge.