Preheat oven to 350°F (175°C). Grease a 9x13-inch pan.

Stir melted butter and sugar until smooth. Add eggs one at a time, mixing after each.

Mix in vanilla. Sift cocoa, flour, cinnamon, cayenne, and salt into the bowl. Fold until just combined.

Stir in chocolate chips if using. Pour batter into the pan and spread evenly.

Bake 25–30 minutes, or until a toothpick comes out with a few moist crumbs. Cool before cutting.