Preheat the oven to 350°F (175°C) and grease an 8×8-inch baking pan or line it with parchment paper.

In a saucepan over low heat, melt the butter until liquid. Stir in cocoa powder until smooth.

Mix in sugar, then add eggs one at a time, whisking well after each addition. Stir in vanilla extract.

In another bowl, whisk together flour and salt. Gradually fold this into the wet ingredients until just combined.

Pour batter into the prepared pan and spread evenly. Bake for about 20-25 minutes or until a toothpick comes out with moist crumbs.

Allow cooling completely before cutting into heart shapes or squares.