Freshly ground coffee can go directly into shortbread dough; beating it along with the butter and sugar will spread the flavour further, and you'll still get little bits of coffee in each bite, along with dark chocolate.
Preheat the oven to 325˚F.
In a large bowl, beat the butter, icing sugar and Co-operative Coffee for 2-3 minutes, until pale and light.
Add the flour, cornstarch and salt and stir by hand or beat on low speed, adding the chopped chocolate as you stir, just until the dough comes together. (It will look dry—you may have to get in there and scrape down the sides of the bowl, or get in there with your hands.)
Roll the dough into walnut-sized balls, then squeeze them to make an oval shape.
Place on a parchment-lined sheet and press down the middle with a bamboo skewer or chopstick to make it resemble a coffee bean.
Bake for 12 minutes, or until pale golden around the edges. Makes about 1 1/2 dozen cookies.