Preheat oven to 350°F. Line an 8-inch square baking pan with parchment paper, or grease with non-stick cooking spray.
Melt butter on high power in the microwave or in a saucepan over medium heat on the stove.
To a large mixing bowl, add melted butter, sugar, eggs, and vanilla extract. Whisk well to combine.
To the butter mixture, add cocoa powder, flour, salt, and baking powder. Stir to combine, until a thick batter forms.
Pour brownie batter into prepared baking pan, and use a knife or offset spatula to spread into an even layer. Bake at 350°F for 20-22 minutes, or until a toothpick inserted into the center of the brownies comes out dry, or with a few moist crumbs.
Remove from oven and transfer pan to a wire cooling rack. Let the brownies cool for 20 minutes, then use a cookie cutter to cut heart shaped brownies out of the pan.
Notes
For festive flair, feel free to stir up to ½ cup of chopped nuts, chocolate chips, or candies like red, pink, and white M&Ms into the batter for your Valentine's brownies.
For easier cutting: If your baking pan has high sides, I recommend removing the cool brownies from the pan before cutting the heart brownies.
The easiest way to remove brownies from the baking pan is to line the pan with parchment paper. Cut the paper long enough that it hangs slightly over the sides of the pan. Simply hold the edges of the paper in your hands and use it to gently lift the entire batch up and out of the pan.