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

Melt ½ cup of unsalted butter until completely liquid, then stir in ½ cup of dark brown sugar and ½ cup of granulated sugar until smooth.

Add 2 large eggs and 1 teaspoon of vanilla extract, mixing vigorously until fully integrated and shiny.

In a separate bowl, whisk together ½ cup of all-purpose flour, ½ cup of unsweetened cocoa powder, ¼ teaspoon of salt, and ¼ teaspoon of baking soda.

Fold the dry ingredients into the wet mixture gently with a spatula until just combined.

Stir in ½ cup of semisweet chocolate chips and any additional mix-ins if desired.

Pour the batter into the prepared baking pan and smooth the top with a spatula.

Bake for 25-30 minutes, checking for doneness with a skewer coming out with a few moist crumbs.

Cool in the pan for 15-20 minutes, then lift out and cut into squares.