Alfajores 7 Steps
Place can of condensed milk in a large pot and cover with 2 inches of water. Bring to a simmer and cook for 3 hours, topping up with water as needed to keep can covered. Don’t let water level recede or can may burst. Drain water from pot and allow can to cool completely before opening.
In a medium bowl, cream butter and 40 grams icing sugar with a hand mixer until light and fluffy. Add vanilla and mix until incorporated.
In a separate bowl, sift together cornstarch and flour. Add cornstarch mixture to butter and stir gently with a spatula to combine. Using your hands, gently knead dough until it comes together. Shape dough into a flattened disk and wrap in plastic wrap. Chill dough in fridge for 30 minutes.
Preheat oven to 350°F. Roll dough between two sheets of parchment to 1/4"-inch thick. Using a small, round cookie cutter, cut dough into rounds and transfer to a parchment-lined tray, leaving a 1/2"-inch space between cookies. Transfer tray to fridge to chill before baking.
Bake cookies for 10-11 minutes, until set and just starting to brown around edges. Cool on a wire rack.
Spread half of cookies with cooled dulce de leche and sprinkle with sea salt. Top with remaining cookies. Sift remaining icing sugar over cookies.
Sandwiched cookies are best eaten the day they're made. Unfilled cookies can be kept in an airtight container at room temperature for 3-4 days. Store leftover dulce de leche in the fridge for up to 1 month.