Combine the butter, cocoa powder, Coca-Cola, and buttermilk in a small saucepan and stir. Bring to boil over medium-high heat, stirring occasionally to prevent cocoa from scorching.
Meanwhile whisk together gluten free flour, sugar, salt, baking soda and cinnamon in a medium bowl.
Once Coca-Cola mixture starts to boil, pour it into flour mixture.
Add eggs and vanilla and whisk until combined.
Spread batter into prepared pan(s). This recipe will make a 13x9" pan or 2 dozen cupcakes.
Bake for 25-30 minutes for cake pan, 12-20 minutes for cupcakes until a toothpick inserted in center comes out clean. Be careful not to over-bake.
Set pan on cooling rack for five minutes, then start the glaze.
FOR THE GLAZE: combine butter, cocoa, and Coca-Cola in a saucepan. Stir to combine and bring to boil over medium-high heat. Remove from heat and whisk in powdered sugar.
Drizzle the glaze over the cake or cupcakes.
Store leftovers in an airtight container on the counter for 3-4 days if they last that long! In my opinion, these taste even better after sitting for a bit.