Beat the room-temperature butter in the bowl of a stand mixer for several minutes, until smooth and creamy.
Add the ½ cup confectioners-style sugar substitute and continue beating until the butter mixture becomes light, fluffy, and thoroughly combined.
Crack in the egg and pour in the mint extract. Mix for another 1–2 minutes, or until the ingredients form a smooth, uniform mixture.
In a separate mixing bowl, stir together the almond flour, unsweetened cocoa powder, and salt.
Gradually incorporate the dry ingredients into the butter mixture while the mixer is running. Continue beating until a dough begins to form.
If the mixture remains overly dry or does not hold together, add water 1 tablespoon at a time until the dough reaches a cohesive consistency.
Preheat the oven to 325°F. Line baking sheets with parchment paper.
Shape the prepared dough into balls measuring approximately 1 inch in diameter and arrange them on the lined baking sheets.
Bake for 12 minutes, or until the cookies feel firm when gently touched. Remove them from the oven and allow them to cool for approximately 5 minutes.
Pour about ½ cup of the confectioners-style sugar substitute into a shallow bowl. Roll the slightly cooled cookies in the sweetener until evenly coated.
Leave the coated mint-chocolate cookies to cool completely before serving.