Bundt Coffee Cake

1 1/2 sticks of butter or margarine, room temperature
1 1/2 c. granulated sugar
3 eggs
1 tsp. vanilla
1/2 c. sour cream
1/2 c. buttermilk
2 1/2 c. all purpose flour
1 tsp. baking soda
1 1/2 tsp baking powder

filling: 1/4 c. flour, 2 tsp. cinnamon, 4 T butter, 1/3 c. brown sugar

Cream butter and sugar together until light and fluffy. Add eggs, blending with hand mixer or stand mixer one at a time. Add vanilla. Add sour cream. Combine flour, baking soda and baking powder. Blend in one cup of the flour mixture, then half of the buttermilk. Blend in remaining flour and milk. Generously butter a 10' bundt cake pan. Lightly coat the inside of the pan with additional cinnamon and sugar. Combine filling ingredients. Pour half of the batter into the pan and sprinkle filling over the top; then cover with remaining batter. Bake in a preheated oven at 350 degrees for 40 mins to one hour(some ovens vary), or until done and golden brown on top. Best served warm.