VANILLA BEAN DULCE DE LECHE FILLING

1. Peel the wrapper off a can of condensed milk, and place it on its side in a large pot. Cover with water by at least 2 inches. Bring the water to a boil, then reduce to a simmer. Simmer the can for 3 hours, checking often to make sure the water is well above the level of the can, and topping up when necessary. Remove from the pot with tongs and allow to cool completely.

CHOCOLATE SHORTBREAD

1. Preheat the oven to 350˚f / 180˚c.
2. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, powdered sugar, salt, and vanilla bean paste until light and fluffy, 4-5 minutes. Add the sifted flour and cocoa powder, and mix just to combine. 
3. Dump the dough out onto a large piece of parchment, and place a second piece over the top. Roll the dough out between the two pieces of parchment, until is is approx ¼" (6mm) thick. Place the dough on a baking sheet, and place into the freezer for 15-20 minutes, or until very firm.
4. Remove from the freezer. Line two baking sheets with parchment paper. Remove the top piece of parchment, and cut 2" (5cm) circles using a fluted cookie cutter. Place them on the baking sheet, with a little space between. Place the cut-out cookies in the freezer for 10 minutes, then, using a small cutter in your desired shape, cut out holes in the centre of half the cookies. Re-roll scraps as needed, freezing briefly before cutting out.
5. Bake the cookies, one tray at a time (or two if your oven is good), for 11-12 minutes, or until set. Remove from the oven and allow to cool completely on a wire rack.

ASSEMBLY

1. Pair the cookies so each pair has a solid cookie, and a cookie with a cut-out. Open the cooled can of Dulce de Leche, and transfer to a small bowl (be careful not to scrape out completely as sometimes sugar gathers on the bottom of the tin). Add the vanilla paste, and mix well until combined and smooth. Transfer to a small piping bag, and snip a small hole in the end.
2. Pipe a small blob onto one half of the cookie pair (the solid half), and top with a cut-out half. Press carefully to sandwich. Repeat with the remaining cookies. Best eaten on the day they are made - if you would like to prepare ahead, store the filling and cookies separately until ready to serve.