Preheat oven to 375°F. Line muffin pans with paper baking cups. (This recipe makes 24-28 muffins. Half the recipe for a smaller batch!)
Make the topping by stirring together all topping ingredients with a fork in a medium sized bowl. Set aside.
Using a hand mixer, mix eggs with electric mixer about 1-2 minutes until thick and frothy. Mix in sugar and oil, beat until creamy. Mix in lemon zest, juice, vanilla, baking soda, salt. Mix in sour cream then stir in flour using a wooden spoon. (Batter may be slightly lumpy.)
Fill muffin cups ¾ full. Sprinkle the tops with the crumble mixture. (I filled mine just a teeny bit too much. They got scary close to overflowing, so stick with 3/4 full!)
Bake for 18-22 minutes until a toothpick comes out clean from the center of a muffin. Cool 5 minutes in pan then remove to a rack to cool completely.
(The glaze is optional but adds another punch of lemon flavor.)
Make the glaze: whisk powdered sugar and lemon juice. Drizzle over muffins.
Muffins should be kept in an airtight container for up to 3 days or freeze them for up to one month. (If freezing, omit glaze and make it before serving.)