Adapted from Baker By Nature
Make the butter topping by combining flour, sugar, cinnamon, and salt in a mixing bowl. Stir in the melted butter, mixing just until combined. Set aside.
Make the cinnamon swirl in a small mixing bowl by combining the sugar and cinnamon. Set aside.
Preheat oven to 200°C and prepare the muffin tin with liners.
Cream together the butter and sugars and beat until light and fluffy. Add in the eggs and yolk, beating well after each addition. Slowly beat in the vanilla, buttermilk, and sour cream. Mix in the flour, baking powder, cinnamon, and salt and mix until just combined.
Scoop batter into each muffin cup. Evenly divide the cinnamon sugar on top of the batter, then evenly distribute the crumble topping making sure to pile it high in the center of each cupcake.
Bake for 20 to 22 minutes, or until the tops are lightly golden brown and a toothpick inserted in the middle comes out clean.