Gluten Free Texas Sheet Cake

I've been making a lot of Texas Sheet Cakes lately. I hadn't ever really tried to make a gluten free version of it until recently. I'm not sure why...perhaps because I'm not a huge huge chocolate cake fan. But one day a few weeks ago I suddenly wanted to make one, so I pulled out my mom's old recipe, compared it to a recipe I found on the web, made some GF adjustments and set to work. The first one was a little crumbly and hadn't been cooked quite enough, but subsequent trials were just lovely. So I thought I'd share.  Enjoy!

Texas Sheet Cake

2 c. EGF all-purpose flour
2 c. sugar
1 tsp. xanthan gum
1 tsp. baking soda
1/2 tsp. salt
1 c. butter (2 sticks)
1 c. water
4 Tbs. cocoa
1/2 c. buttermilk (I use 1/2 c. whole milk plus about 1.5 tsp. lemon juice)
3 eggs
1 tsp. vanilla

In a large bowl, combine flour mix, sugar, xanthan gum, baking soda, and salt. In a medium pan, combine butter, water, and cocoa. Bring to a boil over medium high heat, stirring occasionally. Pour cocoa mixture into flour mixture and mix well until smooth. Add buttermilk, eggs, and vanilla and mix thoroughly. The batter will be quite thin.

Pour batter into a greased (or well-sprayed-with-Pam) 13x18 sheet cake pan. Bake at 400 degrees for 20-25 minutes. (The edges of the cake will pull away from the pan when the cake is done.) Remove from the oven and, while hot, top with the following frosting:

Frosting for Texas Sheet Cake

1/2 c. butter (1 stick)
6 Tbs. milk
4 Tbs. cocoa
1 lb. powdered sugar (about 3 3/4 cups)
1 tsp. vanilla
1 c. coarsely chopped pecans

Combine butter, milk, and cocoa in a medium saucepan and bring to a boil over medium high heat, stirring constantly. Remove from heat and pour over powdered sugar. Add vanilla. Mix well. Add chopped nuts. Pour immediately over hot cake (frosting will firm up quickly and spreads best while still hot).