Preheat the oven to 350°F and prepare a sheet pan lined with parchment paper.
Combine the instant coffee and milk (or water) and stir to dissolve the coffee granules. Set aside until needed.*The instant coffee granules don't have to all dissolve, unless you prefer for the cookies to not have random spots of instant coffee. If you want the instant coffee to dissolve completely, either use 1 teaspoon more water or use hot water (do let the mixture cool completely before adding to the butter mixture).*
In a mixing bowl, cream the softened butter with sugar until light and fluffy, about 2-3 minutes. Add the egg, pumpkin pie spice, vanilla extract, and salt and beat until incorporated.
Add the flour and baking powder to the creamed butter mixture and fold everything together, just until no more dry flour is visible.*You can use the mixer to incorporate the flour, but just make sure to mix on the lowest speed.*
Compact the cookie dough into one ball and transfer the cookie dough onto a clean work surface, lightly dusted with flour. Roll the dough out into a ⅜ inch sheet with a rolling pin and dust with flour as needed. Also, make sure to lift the dough occasionally and dust your work surface as needed to prevent sticking.
With a 3 inch circle cutter, cut out as many cookies as possible. Gently transfer the circle cookie doughs to the lined sheet pan. Compact the leftover cookie dough scraps and repeat.
Bake the cookies for about 10-12 minutes or until the center of the cookies have all puffed up. Remove the cookies from the oven and let them cool completely before removing from the sheet pan.*The cookies will be extremely soft when you take them out of the oven, but they will harden up as they cool.*
For the optional glaze:
Combine the powdered sugar and milk and mix until smooth. Once the cookies have completely cooled, drizzle the icing on the cookies either with a spoon or pipping bag. Before the icing dries, garnish the icing with some slightly sweetened cocoa nibs.
After rolling the cookies out into a ⅜ inch sheet, you can refrigerate the dough for about 15 to 30 minutes before cutting to keep the cookies from having that slightly warped look from handling.
I prefer rolling my dough in between two sheets of parchment for easy handling and keeping it almost mess-free.