Preheat oven to 325 degrees. In a small mixing bowl combine flour, baking powder, baking soda and salt, set aside. In a small bowl mix mashed bananas with lemon juice, set aside. In a large bowl, using an electric mixer, cream together butter and sugar until light and fluffy about 3 - 4 minutes. Mix in egg and vanilla. Add mashed bananas and stir until combine. Mix in the flour mixture alternating with the buttermilk. Pour 1/4 cup mixture into greased muffin cups or paper muffin cups. Bake for 22-24 minutes until toothpick inserted in center of muffin comes out clean. Remove from oven and let muffins cool in muffin pan 5 minutes, then remove from muffin pan and let cool 30 minutes on a wire cooling rack. Pipe vanilla pastry cream on top of muffins and refrigerate until ready to serve. Place banana slices on top immediately before serving.