Preheat the oven to 325F and line a large baking sheet with parchment or a silicone liner.
In a medium bowl, whisk together the almond flour, coconut flour, baking soda, and salt.
In a large bowl, beat the butter with the powdered sweetener until well combined. Beat in the egg yolks, then beat in the brandy, vanilla extract, and lemon zest.
Beat in the flour mixture until the dough comes together. Gather the dough into one big ball and transfer to a silicone mat or a large piece of parchment paper. Shape into a disc and cover with another piece of parchment, then roll out to 1/4 inch thick.
Cut with a 2 inch round cookie cutter and gently lift with an offset spatula. Place on the prepared baking sheet about 1 inch apart. You should get about 36 cookies.
Bake 12 to 15 minutes, until just barely golden. The cookies will still be quite soft but will firm up as they cool. Let cool completely.
Once cool take one cookie and spread the bottom with about 2 teaspoons of chilled dulce de leche. Top with another cookie and sprinkle with powdered sweetener. Repeat with remaining cookies and remaining dulce de leche.