Beat butter and sugar until pale and airy.
Mix in egg and vanilla until smooth.
Whisk flour, baking powder, and salt; add to butter mixture to form soft dough.
Divide dough, wrap, and chill 30 minutes.
Mix melted butter, brown sugar, and cinnamon for filling.
Roll each dough portion into 10×8-inch rectangles, ¼-inch thick.
Spread cinnamon mixture on one rectangle, top with second, and press gently.
Roll into a log, pinch seam, wrap, and chill 15 minutes.
Preheat oven to 350°F; slice log into ¼-inch rounds and place on baking sheet.
Bake 10–12 minutes; cool on sheet 5 minutes, then transfer to wire rack.