• Brownies:Preheat oven to 350. Lightly grease a 9x13 baking dish. I used a 9x13 glass Pyrex.
• Note that you can use a hand mixer or a stand mixer for this recipe. 
• Melt the butter, then add to mixing bowl. Add the sugar and pure vanilla extract and mix until blended.  
• Beat in the eggs, one at a time, mixing well after each addition, until thoroughly blended.
• In a separate bowl, whisk together the gluten-free flour, xanthan gum (if needed), cocoa powder, and salt. 
• Gradually stir the flour mixture into the butter mixture in the mixing bowl until blended. 
• Pour into greased 9x13" pan and place in preheated oven. Bake at 3500 for 30 minutes, or until a toothpick inserted near the center comes out clean or with moist crumbs only, no batter.
• Remove from oven and cool pan on wire rack before frosting.

Frosting:

• Using a medium glass bowl, beat the room temperature butter and the cocoa powder together. Add powdered sugar and milk alternately, beating with a hand mixer until creamy. Add vanilla last, beating in until mixed well.
• Spread over the completely cooled brownies. For nice cutting, chill the completed frosted brownies for at least 30 minutes before cutting.