For a CHOCOLATE sponge cake, you have to replace 100gr of the flour with 200gr of warm melted chocolate. I suggest you mix the chocolate with the flour and then add about 1/3 of the whipped egg mass. When the mixture is well blended (it will slightly follow, but that's completely normal), add it to the rest of the egg mass.
Pls, note that the Chocolate Sponge Cake tends to dry out faster!