Line 2-3 baking trays with silicone mats or greaseproof paper.

In a large bowl (or stand mixer), beat butter and sugars until light and fluffy (about 2 minutes on medium-high).

Add egg yolk and vanilla extract; mix until well incorporated, scraping sides as needed.

Add flour and cornstarch; mix on low until a soft dough forms.

Shape the dough into 1-inch balls and place 2 inches apart on your baking trays.

Flatten each dough ball to 1/2-3/4 inch thick and make an indentation in the middle with your thumb or the end of a small rolling pin.

Cover trays loosely with cling film and refrigerate for 15 minutes.

Preheat the oven to 350 F (175 C).

Bake for 11-14 minutes, or until risen, puffy, and golden around the edges (cookies will still be soft in the center).

Remove from oven and let cool on trays for at least 15 minutes.

Transfer to a wire rack if desired.

Once cool, place about 1 teaspoon of dulce de leche into the indentation of each cookie.

Melt milk chocolate and place in a small Ziploc or piping bag.

Snip a very small piece off the corner and drizzle melted chocolate over each cookie.

Let chocolate set completely before placing cookies in an airtight container.

Store cookies at room temperature for up to 5 days.