Preheat the oven to 375° Fahrenheit.
Using a stand mixer, cream the butter, granulated sugar, and 1/4 cup brown sugar. Mix on medium speed until fluffy.
Add the egg and mix well, stopping once to scrape the sides and bottom of the bowl. Then mix in the buttermilk while the mixer is on low.
Turn the mixer off and add the flour, baking powder, baking soda, salt, and cinnamon. Mix for one minute. Then add the chopped apples. Let rest, then make the streusel topping.
In a separate bowl combine the brown sugar, flour, salted butter, and cinnamon. Using a fork, mix well until small pea-sized balls are formed.
Place cupcake liners in a muffin pan. Divide the batter evenly among the prepared muffin cups. Sprinkle the streusel topping evening among the muffins.
Bake the muffins for 30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven, cool them for 5 minutes in the pan, then turn them out onto a wire rack to finish cooling completely.