Preheat the oven to 350 °F. Line two large cookie sheets with silicone mats.
Cream together sugar and butter in a large bowl. Add the egg and vanilla and mix. Add the baking soda, espresso powder and salt and combine. Mix in the cocoa and finally the oat flour until fully incorporated.
Scoop dough into 1.5 tablespoon portions and place on the prepared baking sheets, spaced apart about 1.5-2 inches (3-4 cm). Press down with the back of a spoon to ½ inch/1 cm height.
Bake for 8-10 minutes until the edges are set but the centres are still soft.
Let the cookies cool on the baking sheet for 5 minutes before moving to a wire rack to cool completely.