Preheat the oven to 400 degrees F. Prepare a 15x10x1 inch sheet cake pan with parchment paper.
Combine flour and sugar in a large mixing bowl. Set aside.
Add butter, water and cocoa powder to a medium saucepan. Bring to a boil over medium-high heat. Combine butter mixture with flour mixture. Stir mixture to combine.
Add eggs one at a time, stirring after each addition.
Pour buttermilk in a measuring cup. Stir baking soda into buttermilk then add buttermilk mixture to the cake batter. Stir in the vanilla and cinnamon.
Pour batter into prepared pan. Bake for 18 minutes. Frost cake immediately after taking it out of the oven.
For the frosting:
Four minutes after the cake has been in the oven, prepare frosting: add powdered sugar to a large bowl. Melt butter (on the stovetop in a small saucepan or in the microwave in a bowl). Add milk, cocoa powder, and vanilla to melted butter, then pour the butter mixture over the powdered sugar. Stir in chopped nuts. Frosting will be thick.
Frost baked cake while hot. The heat from the cake will loosen up the frosting making it easy to spread. Use gentle strokes so the top of the cake stays intact.
Notes
Be sure your baking pan is 15x10x1 for these baking instructions. This is typically the size of large cookie sheets or large jelly roll pans.
You might like a thicker cake. If so, use a 9x13 inch pan and increase baking time until a toothpick inserted in the center comes out clean.