Preheat oven to 350°F (180°C). Grease the 9x13 inch baking dish with butter and line with parchment paper.

In a medium mixing bowl, melt the butter and sugar in the microwave or on the stovetop. Once melted, let cool slightly.

Add the eggs one at a time, whisking well after each addition.

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

Add the dry ingredients to the wet ingredients and stir until just combined.

Stir in the chocolate chips and any optional mix-ins.

Pour the batter into the prepared baking dish and smooth the top.

Bake for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Let cool completely in the pan before cutting into squares and serving.