Combine all of the ingredients in a small bowl using a pastry blender or your fingertips until they have the consistency of wet sand. Set aside until ready to use.
For the Muffins
Preheat the oven to 375F and line or grease 10 cups in a 12-cup muffin tin.
In a large mixing bowl, add the flour, baking powder, baking soda, cinnamon, and nutmeg. Briefly stir together to combine the dry ingredients.
In a medium mixing bowl, combine the sugar, egg, vanilla, vegetable oil, and buttermilk. Whisk until combined.
Pour the wet ingredients into the dry ones and mix until the ingredients are just combined. Do not overmix!
Pour the batter into the muffin tins until they are ¼ full- this should use about half of the batter.
Divide half of the crumb layer between the 10 muffin cups. Divide the remaining half of the batter between the muffin cups and top with the remaining crumb topping.
Bake the muffins for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool on a wire rack. Once cool, store in an airtight container at room temperature for up to two days.
You can swap the Greek yogurt for sour cream for sour cream coffee cake muffins.For a blueberry coffee cake variation, add 1 cup of fresh blueberries to the flour mixture and toss to coat. Then add the milk mixture. You can also use other fresh berries like raspberries or blackberries.