Preheat the oven to 350°F. Line an 8x8-inch square pan with foil and lightly spray it with baking spray. Set aside.
Melt the butter in a small saucepan over low heat. Once melted, stir in the sugar until fully incorporated.
Add the chopped chocolate to the butter mixture, stirring constantly until the chocolate is completely melted and smooth. Remove from heat.
Stir in the eggs one at a time, mixing well after each addition. Then add the vanilla extract and mix until everything is combined.
Add the dry ingredients and beat. Slowly mix in the cocoa powder, cornstarch, salt, cayenne pepper, and cinnamon. Once combined, beat the batter on high speed with a hand mixer for 1 to 2 minutes, until smooth and shiny. (You can also beat it vigorously by hand.)
Pour the batter into your prepared pan and smooth the top.
Bake for 25–30 minutes, or until the center is set. Remove from the oven and allow the brownies to cool in the pan for at least 1 hour.