1. Preheat oven to 400°.
2. To prepare streusel, combine first 5 ingredients in a bowl; set aside.
3. To prepare muffins, weigh or spoon 5 ounces spelt 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.
4. Combine buttermilk, 1/2 cup sugar, oil, 1 tablespoon 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 minutes or until a wooden pick inserted in the center comes out with moist crumbs clinging. Cool in pan 5 minutes. Remove from pan; cool completely.
5. Combine powdered sugar, syrup, and 1/2 teaspoon water in a bowl; stir until smooth. Drizzle evenly over muffins.