Preheat the oven to 350℉/176℃. Line the muffin tin with muffin liners and set aside.
In a medium-sized bowl, add in the dry ingredients. Stir to combine and set aside.
1 1/3 cups all-purpose flour, 1/4 cups cocoa powder, 1/2 tsp baking soda, 1/2 tsp baking powder, 1/4 tsp salt
In a microwave or using a double boiler method, melt the butter and chocolate chips together until smooth. When melted, transfer into a large bowl.
1 1/4 cups semi-sweet chocolate chips, 2/3 cups unsalted butter
Into the bowl with the butter chocolate mixture, add the brown sugar and the white sugar. Stir for 1-2 minutes to combine.
3/4 cups dark brown sugar, 1/4 cup white sugar
Add in the oil. With a whisk, mix until smooth and well incorporated.
1/4 cup oil
* The mixture should be warm at this point, and not hot. If it's hot, allow it to cool for 5 minutes before doing this step.* Add the eggs and vanilla in. Continue whisking until smooth, about 2-3 minutes.
1 tbsp vanilla, 3 eggs
Add the dry ingredients into the wet. Make sure to sift the cocoa powder. With a spatula, stir to combine. A thick brownie batter will form. Don't over-mix.
Mix in the chocolate chips a few times until well incorporated.
3/4 cups semi-sweet chocolate chips
Scoop the brownie batter into each liner, they should be full. Add extra chocolate chips on top if desired.
Bake for 25-35 minutes. If you want gooey middles, check after 25 minutes. For more chewier cups, bake for 30-35 minutes. I baked mine for 32 minutes exactly. Please note that they will continue to bake as they cool from the heat of the pan.
Allow to cool, and enjoy!