Combine flour, salt, sugar and baking soda in a bowl. Cut in the butter with a pastry blender. Work in the lemon zest then mix the egg yolks and vanilla. Shape the dough into 2 balls and chill for 2 hours.

Preheat the oven to 325°F (162°C). On a floured surface, roll out each ball to a ¼” inch .5cm) thickness. Cut into 2” (4.5cm) rounds and transfer to a parchment lined baking sheet. Bake for 15 minutes. Cookies will not brown but should be dry to the touch. Allow to cool completely on a wire rack.

Fill one cookie with a spoonful of dulce de leche, and then place another cookie on top to make a sandwich. Make all of your alfajores in the same fashion.