Preheat oven to 350°F (175°C) and prepare an 8x8-inch baking pan by greasing and flouring it, or by lining it with parchment paper.

Combine melted butter and granulated sugar in a medium bowl, then microwave in 30-second intervals or use a double boiler until fully melted and well combined.

Stir in eggs one at a time, followed by vanilla extract, until fully incorporated.

In a separate bowl, whisk together flour, cocoa powder, and salt.

Gradually add dry ingredients to wet ingredients, mixing until just combined; fold in chocolate chips if using.

Pour batter into the prepared pan and bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs.

Cool brownies completely in the pan on a wire rack, then lift the slab out using the parchment paper overhang.

Use a small heart-shaped cookie cutter to cut individual heart shapes from the brownie slab, re-rolling scraps for additional hearts if desired.