Preheat oven to 350 F and line cookie sheets with parchment paper.

In a large bowl, whisk together the flour, salt, and sifted cocoa powder. Set aside.

In the bowl of an electric mixer, beat the butter on medium speed until creamy, about 1-2 minutes. Add the sugar and mix until combined. Mix in the vanilla extract and egg yolk and blend just until combined. Scrape down the sides of the bowl with a rubber spatula. With the mixer on low speed, beat in the dry ingredients and blend until just combined.

Form dough into balls, using about 1 tablespoon of dough per ball. Roll the balls between your hands an to make them smooth and place them on the baking sheet.

Press your thumb or the bottom of a spherical measuring teaspoon or half teaspoon into each ball to create an indentations in the center. Dip the teaspoon (if using) in flour before using to prevent sticking. Be careful not to press to hard or deep that the dough cracks. 

Bake 12-14 minutes. Remove from oven and use a rounded teaspoon to reform the indentations if needed. Fill them with about 1/2 teaspoon of dulce de leche each. Sprinkle with a pinch of flaky sea salt, if desired. 

Allow cookies to cool on the cookie sheet for five minutes before transferring them to a wire rack to cool.