Place butter in a large bowl and heat in your microwave or on a double broiler until melted. You can also melt the butter in a pot on your stove top. Add your brown sugar and granulated sugar to the butter while it's still hot. Whisk to incorporate.
Add your room temperature eggs and vanilla extract to the butter / sugar mix. Whisk until everything is incorporated.
Add flour, cocoa powder, salt, and baking soda to the wet ingredients along with any add-ins you'd like (I added chocolate chunks). Fold the ingredients together until everything is fully incorporated and you no longer see any dry ingredients.
Pour your batter into your prepared baking dish. Place your baking dish into the center of a preheated 350°F oven. Bake for 25 to 30 minutes, or until a skewer inserted into the center comes out clean of batter, but with a few wet crumbs.
Allow the brownie to cool in the baking dish for 10 to 15 minutes. Transfer to a wire rack to finish cooling. Slice your brownies. Serve plain or top with some ice cream, melted chocolate, chopped nuts, or whatever else you like!