Dissolve the instant coffee in the hot water to make the coffee extract.

Place the butter and generous pinch of salt in the mixer bowl, and mix on medium speed until creamy (using a paddle beater – about 2 to 4 minutes). You can use a stand mixer or a hand-held mixer as well.

Add the caster sugar and instant coffee extract, and mix until the sugar and butter are well mixed, and the mix is creamy – about 1 to 2 minutes.

Add the AP flour and cornstarch (if using) and mix on LOW (or the stir setting), until the flour is mixed in with the butter-sugar mix, and the dough forms clumps. Do NOT over-mix, as this can make the cookies tough and chewy. Add the instant coffee and stir it into the clumps of dough.

Using floured hands OR a spatula – bring the dough clumps together to form a cohesive dough (fold over the dough 2 – 3 times to help bring it together). Then flatten the dough into a disc shape.

Place the dough disc in a gallon-size ziploc bag (about 10 x 10 inches in size), on a flat, smooth surface. Without closing the bag, gently roll the dough using a rolling pin, maintaining an even thickness throughout the dough.

Once the dough is rolled out, seal the bag, and carefully place it on a sheet pan. Then transfer this to the fridge to chill for about 30 – 60 minutes, or until the dough is firm enough to cut through.

When you’re ready to bake the espresso shortbread cookies, line a half sheet pan with parchment paper and preheat the oven to 350 °F/180°C.

Once the oven is preheated, carefully remove the dough disc from the ziploc bag (you may need to cut the bag to remove the dough disc). Place the dough on a parchment paper on a smooth surface, and trim the edges of the dough using a sharp knife.

Cut the dough into 20 (4 x 5) equal pieces. Pierce each piece of cookie with a fork 3 – 4 times.

Place the chilled pieces of espresso shortbread cookies on the parchment paper, with about 1 in of space between each.

Bake the cookies in the center of the oven, for about 15 – 20 minutes, or just until the bottom edges of the cookies are starting to color.

Remove from the oven and allow the cookies to cool. If you're baking both sheet pans at the same time, make sure to switch the sheet pans (between the two oven shelves) halfway through.

Allow the cookies to cool down for a few minutes and then transfer them onto a wire rack to let them cool down completely.

Place a parchment paper-lined sheet pan in the freezer for about 30 minutes.

Place the chopped bittersweet chocolate in a heat-proof bowl. Melt the chocolate over a double boiler, until most of the chocolate is melted, but some clumps of chocolate still remain.

Remove the bowl from the heat and stir gently until all the chocolate has melted.

Remove the chilled sheet pan from the freezer.

Dip half of each cookie in the melted chocolate. Gently shake the cookie to remove excess chocolate. Place the dipped cookie on the lined and chilled sheet pan.

Once the chocolate is set, remove the cookies from the chilled sheet pan, and store in an air-tight container.