In a bowl, combine all the streusel topping ingredients. With your fingers, combine until you have a crumbly mixture. Set aside.
Preheat the oven to 400˚ Fahrenheit. and line a 12-cup muffin tin with paper liners. If you don’t have paper liners, butter and flour the muffin tin.
In a bowl, mix together the dry ingredients (flour, salt, baking powder and baking soda); set aside. In another bowl, combine the oil, brown sugar, and egg. Once combined, stir in the buttermilk and vanilla extract.
Add the wet ingredients to the dry ingredients and mix well. Gently mix in the apple. If the batter seems to thin, add a tiny bit more flour. The batter should be fairly stiff. Spoon the batter into the muffin cups filling them right to the top.
Divide the streusel topping equally among the muffins.
Bake for 15 minutes and then lower the temperature to 350˚ F. and bake for an additional 10 minutes. When the muffins are done, they will spring back when lightly pressed. Otherwise, test the muffins by inserting a toothpick. Let the muffins cool in the pan for 10 minutes and then remove them and let cool for 15-20 minutes on a wire rack before drizzling with caramel icing.
While muffins are baking, prepare drizzle mixture. Combine brown sugar, butter and milk in a medium size microwave-safe bowl. Microwave on high for 30 seconds, stir well and return to microwave for 15 seconds. Stir again.
Add vanilla. While stirring, add powdered sugar and mix until smooth and creamy. Mixture should drizzle easily, but should not be super ‘runny”. Add a bit more milk or powdered sugar to adjust thickness. Drizzle over cooled muffins.
Store the muffins in an airtight container at room temperature.
The calories shown are based on the recipe making 12 muffins, with 1 serving being 1 muffin. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information.**