In a small bowl, combine melted unsalted butter, brown sugar, cinnamon, all-purpose flour, and a pinch of salt. Mix until smooth and refrigerate for about 10 minutes.

In a medium bowl, whisk together flour, baking powder, baking soda, and salt. In a large bowl, beat softened butter with granulated and brown sugar until light and fluffy. Add egg and vanilla extract, mixing well before incorporating dry ingredients until just combined.

Divide the dough in half, spread one half in a bowl, dollop the chilled cinnamon filling over, then spread the remaining dough over it. Fold layers together gently to create a marbled effect.

Shape the marbled dough into balls and place on a lined baking sheet. Freeze for 10 minutes to help maintain shape during baking.

Preheat oven to 350°F. Arrange chilled dough balls on a baking sheet with space between them and bake 10-12 minutes until edges are set but centers are slightly undercooked. Cool on a wire rack.