Step 1Grease 12 muffin tins and heat the oven to 375 degrees.
Step 2In a medium bowl, sift together one-half cup flour with 1 teaspoon mesquite flour and 3 tablespoons each sugar and brown sugar. Using your fingers, mix one-fourth cup plus 1 tablespoon of the butter into the dry ingredients, until the mixture is combined and resembles small peas. Set aside.
Step 3In a large bowl, sift together the remaining flours, sugars, baking powder, baking soda and salt.
Step 4Melt the remaining one-fourth cup butter in a small bowl. In a medium bowl, whisk together the eggs. Whisk in the sour cream, then the milk, vanilla and melted butter.
Step 5Whisk the wet ingredients into the dry ingredients just until smooth. Fold in the apple. Drop the batter into the pan to fill each tin three-fourths full. Sprinkle the streusel topping evenly over each of the tins; you will use all of the topping.
Step 6Place the muffins in the oven, and bake until the topping is golden and a toothpick inserted comes out clean, about 30 minutes. Allow the muffins to cool on a rack for 15 minutes before unmolding. The muffins will keep for 3 to 5 days.