1. Preheat oven to 350 degrees. Grease two 8- or 9-inch round cake pans and dust with gluten-free flour.
2. In the bowl of an electric mixer, beat margarine and sugar at high speed until light and fluffy, approximately 5 minutes. Lower speed to medium and add eggs, one at a time. Add vanilla and beat until fluffy again, scraping down bowl.
3. In a separate bowl or on a piece of waxed paper, sift together flour blend, baking powder, baking soda, xanthan gum and salt.
4. Add half the dry mixture to the sugar mixture and beat at low speed until combined. Add half the milk and blend. Then add remaining dry ingredients and milk. Mix on medium-low speed until smooth, approximately 2 minutes.
5. Divide batter equally between the two prepared pans. Bake in preheated oven for approximately 35 minutes or until done. Cool the cake for 20 minutes in pans. Then turn cake onto wire racks and cool completely before frosting.