Place rack in upper third of oven and preheat to 350° F.

Grease and flour an 8 or 9-inch square baking dish and set aside.

In a medium bowl, whisk together flour, baking powder, and salt in a medium bowl and set aside.

Bring about 2 inches of water to a boil in a medium saucepan.

In a medium heatproof bowl add butter and unsweetened chocolate

Place bowl over the top of, but not touching the simmering water and allow to melt.

Stir to incorporate.

Once melted, remove the bowl from the simmering pot.

Whisk in brown and granulated sugar.

Whisk in eggs, yolk, and vanilla.

Add the flour mixture all at once to the chocolate mixture.

Fold together with a spatula until well incorporated.

Fold in chocolate chips.

Pour batter into prepared pan and bake for 25-30 minutes.

Bake until a skewer inserted in the center comes out clean.

Remove from oven and cool completely.