Preheat your oven to 350F/175 C. Line the bottom of one or two 8-inch (20 cm) cake tins with parchment. Do not butter the sides.
If using one cake tin, you will slice the cake into two layers later.
Whisk room temperature eggs in stand mixer with whisk attachment on medium-high. Add sugar a tablespoon at a time. Add vanilla or vanilla sugar. Continue to whisk until mixture is very fluffy, about 8-10 minutes.
You'll know the batter is ready when you lift the whisk. The batter should drip slowly and maintain its dripped shape on the surface of the batter.
You can also use a hand mixer, but it may take a few minutes longer.
Sieve the flour and baking powder. Add it to the egg mixture in thirds.
Do not overmix or you will lose too much air. The batter will lose some, but should still feel airy.
Divide batter into the prepared tin(s) and bake.
For two tins, bake 17-19 minutes.
For one tin, bake about 20-22 minutes.
When ready, the top should be lightly browned and the center should spring back when gently poked.
Remove from pans and cool upside down on wire rack. Keep oven on for Streusel Topping.
To keep sponges even softer, you can cover in plastic wrap after the cakes have cooled about 20 minutes.
If you baked one cake, slice into two even layers once completely cooled.