Preheat oven to 375.
Line muffin tin with liners.
If you use the no-fade cupcake liners, they're foil on the inside so you need to spray them with some non-stick butter spray.
In a large bowl, cream together butter and brown sugar with an electric mixer.
Add in eggs and vanilla extract; beat until well blended.
In another bowl, combine flour, baking powder, cinnamon, and salt.
Add dry mixture to wet mixture, one third at a time, while alternating with buttermilk.
Beat just until blended.
Fold in chopped apples.
Fill muffin liners about 3/4 full.
Using the empty bowl from the dry ingredients, and mix together the flour, cinnamon, brown sugar, and salt for the topping.
Work the softened butter in with your fingers.
Sprinkle over muffin mixture.
Bake about 20 minutes, or until a toothpick inserted comes out clean.