In a large bowl, add sugar, cocoa and oil. Using a hand-held electric mixer, mix on medium speed until it looks like wet, muddy sand.
150 g granulated sugar, 40 g cocoa powder, ¼ cup vegetable oil
Beat in the eggs, one at a time, until fully incorporated. Then add the mint and vanilla until smooth and glossy.
2 eggs, ½ teaspoon mint extract, ½ teaspoon vanilla bean paste
Add flour, baking powder, coffee and salt; mix on low until just combined and there are no streaks of flour.
120 g all-purpose flour, 1 teaspoon baking powder, ½ teaspoon instant coffee granules, ¼ teaspoon salt
Cover the bowl with clingfilm and chill for at least 4 hours, or overnight. (The dough is very sticky and needs to chill and rest for two reasons; one: it makes it easier to roll the dough into balls, and two: it prevents too much spreading in the oven. So if you don’t want to end up with a crinkle cookie traybake, chill that dough!)
When ready to shape the crinkle cookies, line two trays with baking paper. Working quickly, scoop the dough (heaped teaspoons = ~25 cookies; level tablespoons = ~18). Wet your hands every other roll to help stop the dough from sticking to your hands.
Add a few tablespoons of icing sugar to a shallow, wide bowl. Working with 2-3 balls at a time, roll each ball in plenty of icing sugar, dredging them twice if needed or if the weather is humid, and adding more icing sugar to the bowl as needed. Keep leftover dough chilled as you roll.
~1 cup icing sugar
Space the dough balls well apart on the trays and chill, tray and all for 30 minutes (if you don’t chill them at this point, THEY WILL SPREAD WHEN BAKED!).
Preheat the oven to 170°C.
Bake the Chocolate Mint Crinkle Cookies: 10 minutes for small cookies and 12 minutes for larger ones. They’re ready when the surface has crinkled, the edges are crispy and the centres are still slightly soft.
Crinkle cookies will keep for 5 days stored at room temperature in an air-tight container.