It might be similar to this.
3 eggs
2/3 cup sugar
1/3 cup cornstarch
2 tablespoons cocoa powder
1 tsp baking powder
7 tablespoons butter
¾ cup powdered sugar
1 tsp vanilla
1 egg yolk
Beat the eggs and sugar in a mixer for several minutes until they became a pale yellow.
In a small bowl, sift together the cocoa powder, cornstarch, and baking powder. Pour this into the egg mixture and combine by hand until fully incorporated.
Pour batter into a baking sheet that has been lined with parchment paper. Pour evenly to create a rectangle shape.
Bake at 450 F for 5-10 minutes until a Swedish toothpick comes out clean after poking the center. After removing from oven, immediately flip over on another piece of parchment that has been covered in powdered sugar. Spray or brush the top with water to loosen the cake from the parchment, then slowly peel it off.
In the mixer, beat the butter and sugar until light and fluffy. Add in vanilla and egg yolk, and beat completely.
Spread the frosting over the top of the cake (this works best with a metal knife rather than a rubber spatula). Roll up the cake, then put into the fridge for an hour or two with the seam at the bottom.
