Using a stand mixer with the paddle attachment, begin creaming the butter and granulated sugar until light and fluffy (2 - 4 minutes on medium speed). Scrape down sides of bowl as needed. Note: a hand mixer may also be used
½ cup Unsalted Butter, ¾ cup Granulated Sugar
Add egg yolk and espresso/water mixture to the bowl. Mix on medium speed until well combined. Scrape down sides of bowl as needed.
1 large Egg yolk
Add the flour mixture and mix on low speed until combined.
Sugar Coating
Combine 1 cup of granulated sugar with 1 tablespoon instant espresso powder. Rub the mixture together with your fingertips to work the espresso into the sugar.
1 cup Granulated Sugar, 1 tablespoon Instant espresso
Scoop approximately 1½ tablespoons of cookie dough and roll into a ball. Roll in the sugar/espresso mixture until well coated.
Bake and Cool
Place the cookies onto the prepared baking sheets. Leave approximately 2 inches of space between each cookie. Note: the cookies will spread to almost twice the size.
Bake cookies for 10 - 12 minutes, or until tops and edges of cookies are browning.
Let cookies cool on the baking sheets for 5 to 10 minutes, then transfer to a wire rack to cool completely.
Notes
Storage - place in an airtight container and store at room temperature.