Preheat oven to 350°F. Line a 9-inch (or use an 8-inch) square pan with parchment paper.
Heat butter in a large bowl in the microwave in 30-second intervals, stirring after each interval, until melted.
Whisk in sugars, stirring until smooth, glossy, and mixture runs off whisk in a ribbon fashion when lifted from bowl.
Whisk in eggs, one at a time, beating well after each addition. Whisk in cocoa powder, salt, cinnamon, cayenne pepper, and vanilla until smooth.
Fold in flour, stirring until only a few streaks remain. Pour batter into prepared pan.
Bake 50 to 55 minutes or until brownies are slightly gooey on the inside but the edges are set. Cool completely in pan; slice into 16 squares.