Line a large baking sheet with parchment. Heat the oven to 400°F (200°C). Breathe. You got this.
Make the almond filling. In a bowl, cream the butter with both sugars until smooth and a little fluffy. Beat in the egg, vanilla, and almond extract. Add almond flour, the flour or cornstarch, salt, and citrus zest if using. Mix until it’s a thick, spreadable paste. Pop it in the fridge for 10 minutes while you prep the pastry.
Unfold one sheet of puff pastry on a lightly floured surface. If needed, gently roll to about 10 x 12 inches. Transfer to the lined tray.
Spread the chilled almond filling over the pastry, leaving a 1-inch border all around. Picture a neat frame. It doesn’t need to be perfect.
Lay the second sheet of puff pastry on top. Press the edges together with your fingers, then crimp with a fork. Chill the whole tray for 15 minutes, so the layers stay flaky.
Brush the top with egg wash. Sprinkle with sliced almonds and sugar. Using a sharp knife, lightly score the surface in a crisscross pattern and snip 3 small vents to let steam escape.
Bake 22 to 28 minutes, until puffed and deep golden. If the edges brown too fast, tent loosely with foil and keep going until the center is set.
Cool on the tray for 10 to 15 minutes. Dust with powdered sugar or brush with warm honey for a soft shine. Slice into 8 to 10 squares and serve slightly warm. The first bite crackles a little. That’s the good part.