Decadent Gluten-Free Chocolate Sheet Cake

Preheat the oven to 375°F. Grease a 10x15 inch rimmed sheet pan.

In a large mixing bowl, whisk together the gluten-free all-purpose flour, granulated sugar, brown sugar, baking soda, salt, and cinnamon.

In a separate bowl, whisk the buttermilk, vanilla extract, and eggs together.

In a small saucepan over medium heat, whisk the butter, cocoa powder, and water together until the butter is melted and cocoa powder is dissolved. Remove from heat.

Slowly drizzle the cocoa mixture into the dry ingredients while mixing on low, and blend until just combined. Then, pour in the buttermilk mixture and continue to mix until just combined, not overmixing the batter.

Pour the cake batter into the greased sheet pan and bake for 15-17 minutes, or until a toothpick inserted in the center comes out clean. Remove from the oven and place on a cooling rack.

While the cake cools for about 5-10 minutes, melt the butter in a small saucepan over medium-low heat. Then whisk in the cocoa powder, milk, and vanilla extract.

Slowly incorporate the powdered sugar into the mixture, whisking until combined.

Pour the icing over the still-warm cake and spread it evenly. Allow the cake to cool completely before serving.