Preheat Oven: Set your oven to 350°F (175°C). Lightly grease a muffin pan or line with muffin cups.

Melt Butter: In a medium saucepan, melt the butter over low heat. Remove from heat, then add the granulated and brown sugars, stirring until smooth.

Mix Wet Ingredients: Beat in the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

Combine Dry Ingredients: In a separate bowl, sift together the flour, cocoa powder, and salt. Gradually add this dry mixture to the wet ingredients, stirring just until combined. Do not overmix.

Add Chocolate Chips (Optional): Gently fold in the chocolate chips or chunks, if using.

Fill Muffin Pan: Spoon the batter into the muffin pan, filling each cup about 2/3 full.

Bake: Bake for 18-22 minutes. The brownies should be slightly set but still fudgy in the center. A toothpick inserted should come out with some moist crumbs.

Cool and Serve: Allow the brownies to cool in the pan for 5-10 minutes before transferring to a wire rack to cool completely.