Make the cookies: Whisk the flour, baking powder and salt in a medium bowl. Beat the butter and maple sugar in a large bowl with a mixer on medium-high speed until fluffy, 4 minutes. Add the egg and vanilla; beat until combined. Reduce the mixer speed to medium low; add the flour mixture and beat until just combined. Transfer the dough to a sheet of plastic wrap; shape into a disk. Wrap and refrigerate until firm, at least 1 hour.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Divide the dough into thirds. Working with 1 piece at a time, roll out the dough on a floured surface until 1/8 inch thick. Cut out stars with a 2-inch cutter; arrange 1 inch apart on 2 ungreased baking sheets and sprinkle with gold sugar. Refrigerate the cutouts 15 minutes.
Bake, switching the pans halfway through, until the cookies are set and lightly browned on the bottoms, 13 to 16 minutes. Let cool completely on the baking sheets.
Make the filling: Melt the white chocolate in the microwave in 30-second intervals, stirring, until smooth. Let cool. Stir in the butter with a rubber spatula, then stir in the confectioners¿ sugar, maple extract and lemon juice until smooth. Sandwich between the cookies.