Preheat oven to 350F. Butter a 9x9 dish and line with parchment or foil, then spray. Whisk flour, baking powder, and salt; set aside. Bring about an inch of water to a simmer in a medium saucepan. Place a medium, heatproof bowl on top of saucepan (do not let it touch water) and add butter and unsweetened chocolate. Stir occasionally, until chocolate and butter are melted and smooth.

Remove bowl from saucepan and whisk in both sugars. Whisk in egg, yolk, and vanilla. Add flour mixture all at once, and fold into chocolate mixture with a rubber spatula until combined. Fold in chocolate chips. Pour batter into pan and bake 25-30 minutes, or until a toothpick inserted comes out clean. Cool completely.

Buttercream Frosting  Using an electric mixer, cream butter, 1/4 cup cocoa powder, and salt on medium speed for 3 minutes, then add powdered sugar. Mix on low speed to incorporate sugar, and slowly add milk and vanilla. Gradually increase speed to medium-high, beating until smooth, scraping sides of bowl.

In a small bowl, mix together cream and 3 Tbsp cocoa. Reduce speed to medium, and pour into cream mixture in a slow, steady stream and mix until thoroughly combined. May add more powdered sugar a Tbsp at a time for a stiffer frosting. Spread frosting onto cooled brownies, and cut into squares. Store in an airtight container, or tightly wrapped, at room temperature.