Combine the filling ingredients and set aside. Combine the cereal, flours, sugar, baking powder and xanthan gum in a bowl.
Cut the margarine into the flower mixture until it is in pea size pieces. Add in the milk and vanilla and mix until just combined.
Spread half the batter into a greased 9-inch square baking pan. Sprinkle the filling on top. Spread the second half of the batter on top and bake for 40 - 45 minutes at 350ºF.