In a large microwave safe bowl, stir together the first quantity of butter and the chopped chocolate. Heat in the microwave until melted together, stirring every 20-30 seconds to avoid scorching the chocolate.
In a separate bowl, combine the dark brown sugar, eggs, and vanilla extract. Whisk together until evenly combined. Then, slowly stir the chocolate and butter into the egg mixture. Temper the warm chocolate into the egg mixture to avoid making scrambled eggs in your brownies.
In another bowl, whisk together the flour, cocoa powder, baking powder, and salt. Then, combine the flour mixture into the chocolate mixture. Mix in about 2/3 of the semi sweet chocolate chips.
Pour the brownie batter into your prepared baking sheet, and sprinkle the remaining semi sweet chocolate chips on top. Bake for 18-20 minutes, or until an inserted toothpick can be removed without pulling up batter.
While the brownies are baking, it's time to whip up the buttercream. Add the butter, salt, powdered sugar, and vanilla to the bowl of a stand mixer fitted with the paddle attachment. Carefully mix until the powdered sugar is combined enough to not fly everywhere. Pour in the heavy cream, then turn to medium-high speed and whip until light and fluffy (about 5 minutes). Add in a few drops of pink or red food coloring if you want the frosting to fit the Valentine's Day theme. I chose pink!
Let the brownies cool for 20-30 minutes before frosting them. Sprinkle with Valentine's themed sprinkled if desired, and cut into 12 pieces.