Position racks in the upper and lower thirds of the oven and preheat to 375 degrees . Line 2 large cookie sheets with parchment paper. Using an electric mixer, beat the butter, brown sugar and 2 tablespoons espresso powder at high speed until fluffy. Beat in the egg, baking powder, vanilla and 1/4 teaspoon salt at medium speed. Beat in the flour and 1/2 cup confectioners' sugar at low speed.
Roll the dough into 1-inch balls and place 2 inches apart on the prepared cookie sheets. Using a glass with a flat bottom, flatten the balls 1/3 inch thick. Bake, switching and rotating the pans halfway through, until just soft in the center, 12 to 15 minutes.
Meanwhile, stir together the remaining 3/4 cup confectioners' sugar, 1/4 teaspoon espresso powder, 1 pinch salt and the whiskey. Using a pastry brush, coat the hot cookies with the glaze; press a chocolate espresso bean in the center of each cookie, then transfer the cookies to a rack to cool completely.