In a bowl, whisk together all-purpose flour, almond flour, baking powder, and salt until combined.

In a separate bowl, beat cubed cold unsalted butter and granulated sugar until pale and creamy, about 2 to 3 minutes. Do not overbeat.

Add egg yolk and vanilla extract, mixing until just incorporated. Stir in zest if using.

Fold the dry ingredients into the wet until a shaggy dough forms. Turn onto a lightly floured surface and press into a disk. Wrap and chill for at least 1 hour until firm.

Preheat oven to 350°F (175°C). Line two baking sheets with parchment.

Roll dough between two sheets of parchment to about 1/8 inch thickness. Use a 2 to 2 1/2 inch round cutter for bases and a smaller cutter for the tops with a window.

Transfer cookies to prepared sheets and chill in the freezer for 10 minutes to help them hold shape.

Bake cookies for 10 to 12 minutes, until edges are lightly golden and centers look set.

Let cookies cool on the sheet for 5 minutes, then transfer to a wire rack to cool completely.

In a medium heavy-bottomed saucepan over medium heat, melt granulated sugar, stirring gently until it liquefies and turns an amber color — watch closely to prevent burning.

Remove from heat and carefully add butter, stirring until melted. Slowly pour in warm heavy cream while stirring; the mixture will bubble vigorously.

Return to low heat and stir 1 to 2 minutes to smooth.

Remove from heat, stir in flaky sea salt and vanilla. Let cool until slightly thickened but still pourable, about 10 minutes.

Spoon or pipe a small mound of caramel onto the solid cookie bases, leaving a 1/8 inch border so caramel does not squeeze out.

Top with the cutout cookies and press gently.

Dust tops lightly with powdered sugar and finish with a tiny pinch of flaky sea salt on each cookie.

Allow assembled cookies to rest at room temperature for 30 minutes so the caramel sets.