In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, and salt until well combined. Set this dry mixture aside.

In a large bowl using a hand mixer, beat the softened butter and granulated sugar on medium speed until pale and fluffy, about 3 minutes.

Add the egg yolks one at a time, beating well after each addition. Pour in the pure vanilla extract and mix until fully incorporated and smooth.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms. Do not overmix; if the dough is too crumbly, gently knead it with your hands until it comes together.

Form the dough into a disk, wrap it tightly in plastic wrap, and refrigerate for at least 30 minutes to make rolling easier and prevent spreading in the oven.

Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.

On a lightly floured surface, roll the chilled dough out to about 1/4-inch thickness. Use a 2-inch round cookie cutter to cut out circles and place them on the prepared baking sheets, leaving 1 inch of space between each cookie.

Bake for 10 to 12 minutes, or until the cookies are set and just barely starting to turn golden around the bottom edges. Let them cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.

To assemble, spread about 1 tablespoon of thick dulce de leche on the flat side of one cookie and gently press another cookie on top to create a sandwich. Roll the sticky edges in shredded coconut and serve with a hot cup of coffee or tea.