For the almond custard filling:

Using a food processor or mixer, beat the butter and sugar together until creamy and light. Be sure this is mixed well together. Add the almond flour and salt and beat well. Mix in one whole egg and one egg white only. Reserve the egg yolk for the glaze. Add both the vanilla and almond (if using) extract. Cover and chill until ready to assemble.

For the galette:

Line a baking sheet with parchment paper. Place one circle of puff pastry dough on top. Carefully spread the almond filling evenly across the dough leaving a one inch. Dampen the edge of the first circle and lay the second circle over it.

Seal the galette together by pressing the edges with your fingers. Using the bottom of the butter knife, recreate a scallop edge by pushing the dough in about 1/4 inch. Refrigerate the galette for 30 minutes before baking to get the puff pastry chilled.

Preheat the oven to 210C/410F. Use a pairing knife to decorate the top by etching a criss-crossing or leaf pattern. Brush a thin layer of yolk glaze over the top. Bake for 30 minutes until the galette is golden and puffy. Remove from oven and let it rest for 15 minutes. Serve warm or at room temperature.