Preheat the oven to 375˚F. Line muffin cups with paper baking cups. Spray inside of muffin cups with cooking spray.
In a medium bowl, combine flour, sugar, baking powder, baking soda and salt. Mix well.
In a small bowl, mix together the sour cream, lemon juice, oil, lemon peel and egg until fully combined. Add to the flour mixture and stir with a fork until the flour mixture is moistened.
Scoop the batter into the paper-lined muffin cups.
In a small bowl, mix together the graham cracker crumbs, brown sugar and butter, then sprinkle on top of the muffins.
Bake for 15 minutes or until a toothpick inserted into the center of the muffin comes out clean. Remove and let cool on a wire rack.