The combination of chocolate and chile (a pairing as old as the chocolate-chile drink that the Aztecs called xocolatl) makes these "cookies for grown-ups". Even if they're too intense for tender young palates, the combination of  chocolate richness plus chile heat is a highly entertaining to adult tastes.

You could tone these down by using ground ancho chile instead of cayenne. Or ramp them up: even with a teaspoon of cayenne, they're not ridiculously hot.

1-1/2 cups all purpose flour
1/2 cup dark cocoa powder
1/2 to 1 teaspoon ground cayenne or more, to taste
2 teaspoons ground cinnamon
1 teaspoon baking soda
1/2 teaspoon salt

1 cup butter (2 sticks)
1-1/2 cups light brown sugar
1 egg
2 teaspoons vanilla extract

Bring butter and egg to room temperature.

Add the dry ingredients (flour, cocoa powder, cayenne, cinnamon, baking soda & salt) to a bowl and whisk to combine.

Cream the butter and brown sugar with a mixer on medium high, until fluffy and light color, about 2-3 minutes.

Add egg and vanilla to the butter/sugar, then mix to combine.

Add the dry ingredients and mix to form a soft dough.

Form balls of dough, each using about 3-4 tablespoons of dough. Place on parchment lined cookie sheets.

Place the cookie sheets into a refrigerator for about 20 minutes.

While the dough is chilling, preheat the oven to 350F

Bake at 350 for 10-12 minutes, in batches if necessary depending on the size of your oven. 

Cool completely.