To make the Mexican chocolate custard base, in a pot whisk together the cream, half and half, brown sugar, cocoa powder, cinnamon, and cayenne until very well combined. (The cocoa powder may form lumps so just keep whisking until they’re gone.) Add the chocolate chips and while occasionally stirring, cook on medium-low heat until warm and the chocolate chips have melted, about 4-5 minutes. Do not let it come to a boil.
While the base is heating, beat the egg yolks with the vanilla. Once the base is warmed, pour 1/4 cup of the base into the beaten eggs, stir until well combined, and then pour the egg mixture into the pot.
While stirring, continue to heat on low for 3 minutes or until the base gets slightly thick. Do not let it come to a boil. It’s ready when it coats the back of your spoon. Cool covered in the refrigerator for at least 4 hours.
While the custard is cooling, make the brownies. Preheat the oven to 350° F and either line a 4-inch oven-proof skillet with parchment paper, or line 6 cups in a regular-sized muffin tin with parchment cups.
In a pot, on low heat melt the chocolate and butter, while occasionally stirring. Stir in the sugar, ancho chile powder, cinnamon, salt, cayenne, and vanilla extract. Mix in the eggs until well combined, and then add the flour and pecans, stirring until a smooth, thick batter is formed.
If using a skillet, spoon the batter into the pan. If using a muffin tin, fill each hole 2/3 full. Bake uncovered for 20-25 minutes, or until the edges are set. Allow to completely cool for at 30 minutes before cutting the brownies into about 1/2-inch bite sized pieces. Store in the refrigerator until time to churn the ice cream.
After everything has chilled, prepare the custard per your ice-cream manufacturer’s instructions. When the ice cream has churned, spoon it into a freezer-safe container then gently stir the brownie pieces into the ice cream. You can serve the ice cream immediately but it’s better if you let it harden in the freezer for at least 4 hours.