Whip 4 eggs in a stand mixer on high speed until foamy (~2 min) then gradually add 1 cup of granulated sugar 1 tablespoon at a time, while continuing to whip. Then, whip until the mixture falls off the whisk in a thick ribbon that doesn't disappear for 1-2 seconds (~20 min; or more if using a hand mixer).

Meanwhile, preheat the oven to 350F with the rack centered.
Prepare a 8 inch round baking tray. Line with foil or parchment and spray with nonstick spray or butter on the inside of the pan, then dust it with flour, knocking out the excess to make the surface non-stick.

Sift 1 cup all-purpose flour and 1 tsp baking powder, twice.

In a cup whisk together 2 tblsp melted butter, 1 tblsp hot water and 1 tsp of vanilla extract.

Add the dry ingredients into the whipped eggs in halves, carefully folding with a spatula forming the number 8 with each stroke. Do not overfold or the cake will be dense and doughy. Just fold enough to barely incorporate the flour.

Do not overmix or the cake will be dense.

Pour the sponge cake batter into the prepared baking pan. Bake for about 35 minutes or until a toothpick inserted into the center comes out clean. Do not open the oven to check until you can smell cake in the kitchen (~30 min).

Remove the cake to a cooling rack and allow to sit undisturbed for 5 minutes. Then invert it onto a cooling rack and remove the foil or parchment. Allow to completely cool before serving.