In a bowl, whisk together the flour, baking powder, salt, cinnamon, and brown sugar.
In a separate bowl, whisk together the buttermilk, egg, vanilla, and melted butter.
Stir the dry ingredients into the wet ingredients and mix until just combined.
Fold in the apples.
In a food processor, combine the oats, brown sugar, chilled butter, and pumpkin seeds. Pulse to combine and chop the pumpkin seeds.
Pour batter into greased muffin cups, top each with 2 teaspoons of the pumpkin seed mixture, and bake 20-22 minutes, or until a wooden pick inserted in the center comes out clean.