Preheat oven to 350˚F.
In a mixing bowl, whisk together flour, cocoa powder, baking soda, salt, and cinnamon. Set aside.
In a stand mixer fitted with a paddle attachment or with a hand mixer cream together butter and sugars. Add melted chocolate and continue to cream together.
Add eggs one at a time, scraping down the sides of the bowl after each addition. Add milk and mix until just combined.
Add flour mixture and stout to butter mixture, ⅓ at a time alternating, beginning with the flour mixture.
Scrape down sides of the bowl and mix until everything is well incorporated and fold in ground Mexican chocolate. Pour mixture into a prepared 9”x13” pan and bake for 35 to 40 minutes or until a toothpick comes out clean when inserted into the center of the cake.
Allow cake to cool completely before frosting, cutting and serving.
For frosting: Place butter, and peanut butter in a mixing bowl and beat together with a hand mixer. Add powdered sugar, 1 cup at a time, until fully incorporated. Stir in cream and vanilla until frosting is smooth and creamy.
To assemble: Spread frosting over surface of the cooled sheet cake and top with a sprinkle of grated Mexican Chocolate. Serve.