Set up a saucepan with water and a can of sweetened condensed milk on the stove.
Make sure the water fully covers the can by a few inches and bring to a boil. Reduce the heat to a simmer, after a boil is reached, and let it cook for 2-3 hours on the stove.
Prep remaining ingredients once dulce de leche is done cooking. Dulce de leche will cool while you prep and form the cookies. To a food processor, add flour, cashews, sugars, and salt, and blend until nuts are finely chopped.
Cut cold butter into cubes and gradually pulse into the flour mixture. Once the dough starts to come together, add egg yolk and baker’s extract and pulse until just combined.
Preheat oven to 350°F.
Remove dough from the processor and shape into 20 cookie dough balls and place on a parchment-lined baking. Make sure to leave enough space for cookies to spread after pressing the thumbprint before the second bake.
Bake for 5 minutes, remove, press a thumbprint (or teaspoon) into cookies, and fill with dulce de leche and flake salt. Return to the oven for another 5 minutes or until lightly browned and remove. Let cool for 5-7 minutes to set before removing from the pan.
Tips
If you would like a thicker, deeper-flavored dulce de leche, cook for a longer time frame. You can make multiple cans at a time. If kept unopened, it has a 3-month shelf life under refrigeration.
Wear heat-proof gloves to press the thumbprint to avoid burning if needed, as the cookie dough will be hot
