Prepare the pan: Grease and line a 9x13 inch baking pan (or a pan of similar size) with parchment paper, leaving an overhang for easy removal. Note: For individual heart brownies, you can bake the brownies in a regular pan and then use a heart-shaped cookie cutter after they've cooled.
Combine wet ingredients: In a large bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until well combined.
Combine dry ingredients: In a separate bowl, whisk together the flour, cocoa powder, and salt.
Combine wet and dry ingredients: Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just combined. Do not overmix.
Add chocolate chips (optional): If using chocolate chips or chunks, gently fold them into the batter.
Bake: Pour the batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with just a few moist crumbs attached. Be careful not to overbake.
Cool: Let the brownies cool completely in the pan.
Cut out hearts (if using a large pan): Once the brownies are completely cooled, use a heart-shaped cookie cutter to cut out individual brownies. If you baked in individual molds, skip this step.
Make the frosting (optional): While the brownies are cooling, prepare the frosting. In a large bowl, cream together the softened butter and powdered sugar until light and fluffy. Gradually add the milk or cream and vanilla extract, beating on medium speed until light and fluffy. Add food coloring to achieve your desired shade.
Frost and decorate (optional): Frost the heart-shaped brownies and decorate with sprinkles, chocolate shavings, fresh berries, or other toppings of your choice.