Position a rack in the bottom third of the oven and preheat to 425 degrees . In a small bowl, microwave 4 tablespoons butter and the maple syrup at high power until melted, 30 seconds; stir. In a small bowl, stir together 1/3 cup granulated sugar, the brown sugar and the cinnamon.
In a food processor, combine the flour, baking powder, the remaining 2 tablespoons granulated sugar and the salt; pulse to combine. Cut the remaining 1 stick chilled butter into small pieces and pulse with the flour mixture until coarse crumbs form. Pulse in the buttermilk until the dough just comes together. Transfer to a lightly floured work surface; pat into a 10-inch square (the dough will be sticky). Quarter the dough, then cut each quarter into 16 equal pieces; roll into balls.
Dip each ball in the maple butter, then coat with the sugar mixture. Place 5 to 6 coated balls into each muffin cup and press to compact.
Bake until cooked through and golden, 15 to 17 minutes (tent with foil if the tops darken too quickly). Transfer the muffin pan to a rack and cool for 10 minutes. Using a knife, loosen the sides of the monkey bread; invert onto a platter, then turn right side up. Serve warm.