Preheat your oven to 325 Fahrenheit. Line a baking sheet with parchment paper and set it aside. In a bowl, mix together the almond flour, coconut flour, baking soda, and salt. Set the bowl aside.
To another bowl, add the softened butter and sweetener. Beat using a hand blender for 1 minute until fluffy. Add the egg yolk, heavy cream, and vanilla extract and beat for another 30 seconds.
Add the dry ingredients to the wet and mix until the dough comes together. Cover the bowl with plastic wrap. Place it in the fridge for 15 minutes.
Take the dough out of the fridge and place it between two pieces of parchment paper. Roll out the dough to ¼ inch thick. Use a 2-inch cookie cutter to cut the cookies.
Lift the cut dough pieces using a metal spatula, and gently place them onto the prepared baking sheet. Bake for 12-14 minutes, or until slightly golden around the edges. These cookies will still be soft when you first take them out of the oven but will firm up as they cool.
Once completely cool, take one cookie, and add ½ tbsp of cold Keto dulce de leche in the center. Top with the second cookie, and gently press until the filling reaches the edges. Repeat with the rest of the pieces until you have 12 cookie sandwiches.
To serve, arrange the Keto alfajores on a platter or cutting board and sprinkle the top with powdered sweetener. To store, place them in an airtight container, and keep them in the fridge for up to a week.