Preheat oven to 350 degrees. Butter the bottom and sides of a 16 x 12 cookie sheet.
Combine the butter, water, and cocoa powder in a medium saucepan on a medium heat until until the mixture comes to a boil.
Remove from the heat and cool for 5 minutes.
In a medium mixing bowl, sift together flour, sugar, salt, baking powder, and cinnamon.
Stir the hot butter and cocoa mixture into to the dry ingredients.
Add the lightly beaten eggs, buttermilk, and vanilla and mix until everything is combined.
Pour batter into the buttered cookie sheet.
Bake for 15 minutes or until a toothpick inserted into the center of the cake comes out clean.
While the cake is baking, melt the butter, cocoa, and buttermilk for the glaze in the saucepan used earlier on a medium heat.
Bring to a boil and remove from the heat.
Add the vanilla and powdered sugar and whisk to combine.
When the cake is finished baking, remove from the oven and pour the hot glaze on the hot cake.
Cool to room temperature and cut into slices.