Chocolate sponge with hot chocolate sauce

Ingredients: 1 cup (200g) granulated sugar 1 cup (120g) all-purpose flour 1/4 cup (25g) unsweetened cocoa powder 1 teaspoon baking powder 1/2 teaspoon baking soda 1/2 teaspoon salt 1/2 cup (120ml) buttermilk 1/2 cup (120ml) vegetable oil 2 large eggs, at room temperature 1 teaspoon vanilla extract