2 sticks unsalted butter, plus more for greasing
2 cups sugar
4 large eggs
2 tsp vanilla extract
2/3 cup Dutch process cocoa powder
1 cup all-purpose flour
1 tsp ground cinnamon
1/4 tsp cayenne
1/2 tsp kosher salt
1/2 tsp baking powder
1/2 cup semisweet chocolate chips
Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking dish with parchment paper. Lightly grease the parchment paper with butter.
Melt the 2 sticks of butter in a medium saucepan over medium-low heat (don't brown the butter, just melt it gently). Remove from the heat and let cool slightly (you don't want to scramble your eggs). Add sugar, eggs and vanilla. Stir with a wooden spoon or spatula until combined.
Add the cocoa, flour, cinnamon, chili powder, salt and baking powder and mix until smooth. Pour the batter into the prepared pan and level it out with a spatula. Tap on the counter once or twice to even out the batter and remove any bubbles. Bake until a toothpick inserted in the middle comes out clean, 25 to 30 minutes. Cool completely, then remove the brownies from the pan and slice.