Preheat oven to 375° F, and grease a medium rectangular cake pan. Cream butter and sugar and blend in eggs. Add bananas and vanilla, beating until blended. Add dry ingredients, beating to combine well. Gently fold in yogurt and nuts. Pour batter into the greased pan, and bake for about 45 minutes. Check doneness by inserting a toothpick in the middle of the cake. If it comes out clean, remove cake from oven. If additional cooking time is required, do so in 5- to 10-minute increments until cake is done. Allow to cool, then frost with our White Milk Frosting recipe.