Preheat the oven to 375 and grease a 12-cup muffin pan with some baking spray.In a large mixing bowl, whisk egg, sugar, melted butter, sour cream and buttermilk.Sift in flour, salt, baking powder and baking soda. Whisk together until smooth.Add lemon zest and whisk until evenly incorporated. Set batter aside. Let it stand for about 15 minutes.In a medium bowl, combine brown sugar, white sugar, lemon zest and flour. Mix well until all ingredients are incorporated evenly.Pour in melted butter and carefully fold it in with a large fork. You will want to be careful so you don’t over-mix. Streusel should look like large crumbs and not paste.Sprinkle about two tablespoons of streusel on top of each muffin.Fill muffin pan cups about ⅔ full with muffin batter. I recommend using an ice cream scoop when making muffins and cupcakes, so that you have an even amount of batter each time. Bake for 18-20 minutes. (Do a toothpick test to check if they are done, or just touch the top to feel how firm they are.)