Preheat oven to 350℉. Grease an 8-inch square cake pan with butter or cooking spray; set aside.
Melt butters in a medium saucepan over medium heat. Stir in chocolate until combined and smooth. Remove from heat and allow to cool for 5 minutes.
In a small bowl, whisk together flour and baking powder.
In a large bowl, whisk together eggs, sugar, vanilla, and salt until fluffy and light yellow. Add chocolate mixture and mix until combined. Add flour mixture and mix until combined.
Pour into greased pan and bake for 20-25 minutes, or until the edges pull away from the pan and the top has an even, shiny crust. Let cool for 20 minutes.
Cut into 12 even pieces. Serve warm.