Preheat the oven to 325 degrees F. Line a 13-by-9-inch pan with foil, leaving a 2-inch overhang, and spray with nonstick cooking spray. Toast the ancho chiles in a medium skillet over medium heat, stirring often, until softened and fragrant, about 5 minutes. Let cool completely. Transfer to a food processor and pulse until roughly chopped, then remove 1/4 cup of the chiles and set aside. Add the flour, cocoa powder, cinnamon, baking powder and salt to the bowl of the food processor and pulse until well combined and the chiles are finely chopped.
Heat the brown sugar, butter and 1 cup water in a large saucepan over medium heat, whisking occasionally, until the butter and sugar melt, about 4 minutes. Remove from the heat and whisk in the unsweetened chocolate, then let cool 10 minutes. Whisk in the eggs and vanilla. Add the flour mixture and whisk until smooth, then pour the batter into the prepared pan. Bake until a toothpick inserted in the center comes out almost clean, about 30 minutes. Put on a cooling rack and sprinkle with the bittersweet chocolate, then let sit until soft enough to spread, about 5 minutes. Spread the chocolate in an even layer with a spatula and sprinkle with the reserved chiles and cayenne. Let cool completely, then cut into brownies.
When measuring flour, we spoon it into a dry measuring cup and level off the excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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