Sugar Cookie Bars

Kim had these at Ashli's bridal shower. They were getting rave reviews and since I couldn't try them, I was up to the challenge to create them GF. Derek said he thought they were just as good and maybe a little better than the originals. (He is so kind)
Sugar Cookie Bars
1/2 cup butter
1 cup sugar
3 eggs
1 tsp vanilla
1/2 tsp salt
1/4 tsp soda
2/3 cup brown rice flour
2/3 cup tapioca flour (starch)
2/3 cup cornstarch
1 tsp xanthan gum

Cream butter and sugar. Add eggs and vanilla and cream until light and fluffy. Mix dry ingredients together. Add to creamed mixture. Spread into greased 9x13 pan. To even out dough, put a plastic bag on your hand (I used a sandwhich bag), spray it with PAM spray and pat it gently. Bake at 375 for 15-20 minutes or until lightly browned on top.
Cool and frost:
Frosting:
1/2 cup butter
1 tsp vanilla
2 1/2 cups powdered sugar
4 tbsp milk
Mix together and spread onto cooled bars. Top with sugar sprinkles to add color.