To prepare streusel, combine first 5 ingredients in a bowl; set aside.
To prepare muffins, weigh or spoon 5 ounces flour and whole-wheat pastry flour into dry measuring cups; level with a knife. Combine flours, cinnamon, baking powder, baking soda, and salt in a medium bowl.
Combine buttermilk, 1⁄2 cup sugar, oil, 1 tbsp melted butter, and egg in a bowl. Add buttermilk mixture to flour mixture, stirring just until combined. Fold in apple. Divide batter among 12 paper-lined muffin cups; top evenly with streusel. Bake at 400 for 16 mins or until a wooden pick comes out with moist crumbs clinging. Cool in pan 5 minutes. Remove from pan; cool completely.
Combine powdered sugar syrup, and 1⁄2 teaspoon water in a bowl; stir until smooth. Drizzle evenly over muffins.
Recipe by How to Have it All at https://www.howtohaveitall.net/apple-streusel-muffin-recipe/