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Place chocolate chips in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes. Let cool.
Preheat the oven to 350 degrees F (175 degrees C).
Whisk flour, cocoa powder, baking powder, cinnamon, baking soda, and salt together in a bowl until thoroughly combined. Set aside.
Combine butter and sugar in a bowl and beat using an electric mixer until light and fluffy. Beat in egg and vanilla extract until thoroughly combined. Mix in melted chocolate until thoroughly incorporated. Stir in 1/2 of the dry ingredients, mixing until just combined. Mix in buttermilk. Stir in remaining dry ingredients, mixing until just combined.
Use a medium cookie scoop to drop rounded balls of dough 1 1/2 inches apart on the prepared baking sheets.
Bake in the preheated oven until cookies are just set, 8 to 10 minutes. Do not overbake. Remove from the oven and allow cookies to cool slightly on the baking sheets. Do not remove cookies from the baking sheets.
Combine butter, heavy cream, cocoa powder, vanilla extract, and salt in a saucepan set over low heat. Whisk until butter is melted and frosting is smooth and combined. Mix in powdered sugar 1 cup at a time, whisking thoroughly after each addition. Keep whisking over low heat until frosting is smooth and shiny, 1 to 2 minutes.
Remove chocolate frosting from heat and immediately pour approx 1 to 2 tablespoons over each cookie while still warm. Use the back of a spoon to gently smooth out frosting as needed. Allow cookies to cool and frosting to set completely before gently removing from the baking sheets.