I have always had these 'jinx' when it comes to baking sponge cakes. I don't really know why, though I've followed exactly what the recipe said, but still, it will end up either being heavy or 'very not a sponge cake should look like'
Cover and put in the fridge for 15-30 minutes. Bake on parchment paper or a greased cookie sheet for 6-10 minutes. (6-7 minutes if you like them softer-6 minutes for thinner cookies, 7 minutes for thicker cookies-or8-10 minutes for more firm cookies