Soft-center chocolate espresso cakes

INGREDIENTS

4 tbsp

unsalted butter, at room temperature, plus more for greasing the tin

1/3 cup

granulated sugar, plus more for tin

8 oz

bittersweet chocolate, chopped

1/3 cup

all-purpose flour

1 tbsp

instant espresso powder

1/4 tsp

salt

large eggs

1 tsp

vanilla extract