In a large bowl, whisk together flour, baking soda and salt. In a second, medium size bowl, using a hand mixer, beat together butter and sugar until fluffy. Add egg, vanilla, mashed bananas and sour cream mixing until well blended. Using a spatula or wooden spoon, gently mix in wet ingredients into dry until just blended. Stir in 1/2 cup of granola being careful not over stir.