Preheat oven to 375°F and line a baking sheet with parchment paper and set aside. Pull the butter out of the refrigerator to sit at room temperature 15 minutes before using. In a medium bowl, whisk together the flour, coffee grounds, malted milk powder, salt, and baking soda, and set aside.
Add the egg and additional egg yolk and mix for 30 seconds. Let rest for 3 minutes, then whisk again for 30 seconds. Repeat the resting and whisking for 2 more rounds. Use a rubber spatula or paddle attachment to mix the dry ingredients into the wet ingredients for 1 minute.