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1
Line 2-3 baking trays with silicone mats or greaseproof paper.
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2
Place butter and sugars into a large bowl, or the bowl of your stand mixer, and beat until light and fluffy, about 2 minutes on medium-high speed. Add the egg yolk and vanilla extract and mix until well incorporated, scraping sides as necessary. Add the flour and cornstarch and mix on low, until a soft dough forms.
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3
Shape the dough into 1 inch balls and place 2 inches apart on your baking trays. Flatten each dough ball until it's around 1/2-3/4 inch thick and then make an indentation into the middle with either your thumb or the end of a small rolling pin.
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4
Cover trays loosely with cling film and place in the fridge for 15 minutes.
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5
Whilst the dough is chilling, preheat the oven to 350 F.
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6
Once chilled, place in the oven for 11-14 minutes until, risen, puffy and golden around the edges, cookies will still be soft in the centre. Remove from oven. Leave them to cool on the trays for at least 15 minutes, before transferring to a wire rack if desired.
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7
Once your cookies are cool, take your dulce de leche and place around 1 teaspoon into the middle of each cookie. You may need more or less, depending on how much your cookie will hold!
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8
Once all the cookies are filled, place melted chocolate into a small Ziploc bag/piping bag and snip a very small piece of the corner off (you don't want huge, thick lines of chocolate, so it really is just the smallest amount of the corner you should remove). Drizzle melted chocolate over each cookie, straight, diagonally or whichever takes your fancy! Leave to set before placing in an airtight container. Cookies will keep at room temperature, for 5 days.
