Defrost puff pastry dough the night before making in the refrigerator.
Preheat oven to 415 degrees
To prepare the frangipane or almond filling:
Cream butter and sugar lightly, add egg in portions and finally add flour and ground almonds.
1. using a generous dusting of flour on the counter and on a rolling pin, roll out one sheet of puff pastry dough into an 11 inch square.
2. Using a 10 inch pie pan turned upside down as your guide, cut a circle out of the dough with the tip of a knife. Refrigerate the sheet on a baking sheet lined with parchment paper. Repeat rolling and cutting the second sheet.
3. Pour frangipane filling into the center of first sheet of pastry leaving a 1 1/2 inch margin for the edge and lightly brush edge with beaten egg.
4. Place the second sheet of dough on top of the first and press the edges with a fork very firmly.
5. Lightly brush the top of the gallette with more of the beaten egg. Use a sharp pointed knife, make decorative crisscross patterns on top sheet a few small slits so that steam can vent while baking.
6. Bake for 15 minutes in the preheated oven. Avoid opening the oven while baking as the pastry may not fully puff.
7. Remove gallette from the oven and dust with confectioner's sugar. Return to the oven for 12 to 15 more minutes, and until top is golden brown.
Let cool before serving.