Preheat the oven to 325 degrees.
Line a square baking pan with parchment paper and lightly butter. Heat the butter, chocolate, and Nutella in a medium-sized saucepan over low heat until melted. Whisk in the sugar. Remove from the heat.
Combine the flour, baking powder, cocoa powder, and salt in a large mixing bowl. Add eggs one at a time to the melted chocolate, whisking well after each egg. Add the chocolate mixture to the dry ingredients and stir until combined. Add the vanilla extract and stir. Pour into the baking pan. Bake for 25-30 minutes. Remove from the oven and let cool before slicing.
