Prepare the Pan: Preheat the oven to 350°F. Grease 2 mini muffin tins or a 9 x 13-inch pan, then set the prepared pan aside.
Combine Dry Ingredients: In one bowl, mix together the flour, ground cinnamon, baking powder, and salt until evenly combined.
Prepare Sugar Mixture: In a separate bowl, combine the sugar and unsweetened cocoa powder.
Melt the Butter: Place the butter in a large microwave-safe bowl and melt it in the microwave.
Build the Batter: Stir the sugar and cocoa mixture into the melted butter. Add the eggs one at a time, mixing after each addition until well blended. Stir in the vanilla extract.
Add the Flour Mixture: Slowly incorporate the flour mixture into the wet ingredients, stirring until the ingredients are incorporated and the brownie batter is combined.
Fill the Pan: Scoop the batter into the prepared mini muffin tins or spread it into the prepared 9 x 13-inch pan.
Bake the Brownies: Bake mini brownies for 15 minutes or bake the brownies in the 9 x 13-inch pan for 30 minutes, or until the brownies begin to pull away from the pan.
Cool Completely: Remove the brownies from the oven and allow them to cool completely before adding the frosting.
Beat the Butter: For the frosting, place the softened butter in a mixer and beat until light and fluffy.
Add Chocolate Flavor: Beat in the vanilla extract and melted unsweetened baking chocolate until well blended.
Combine Frosting Dry Ingredients: In a separate bowl, combine the confectioners' sugar and ground cinnamon.
Finish the Frosting: Gradually add the confectioners' sugar mixture to the butter mixture, beating until well blended. Scrape down the sides of the bowl as needed.
Add the Milk: Carefully and slowly beat in the milk until the frosting becomes smooth.
Frost the Brownies: Once the brownies are completely cool, pipe or spread the prepared frosting over the top.
