In a small bowl, combine instant espresso with 1 tablespoon hot water. Stir until completely dissolved. Set aside and let cool.

In a medium bowl, combine flour, baking powder, and salt. In a large bowl, cream together butter and sugars with an electric mixer until light and fluffy. Beat in egg and dissolved espresso until well mixed.

Stir in flour mixture with a rubber spatula and mix well. When a soft dough begins to form, use your hands to finish blending. Mix in ½ cup nuts.

Cut dough in half. Roll out each piece into logs about 2 inches thick. Wrap each log tightly in plastic wrap and place in the refrigerator until firm, about 4 hours or overnight.

Preheat the oven to 375°F. Grease 2 cookie sheets with nonstick spray, or line with parchment paper, and set aside.

With a sharp, thin knife carefully cut logs into ¼-inch-thick slices. Place slices on cookie sheets about 1 inch apart. Press equal amounts of remaining walnuts on top of each slice. Bake until firm and lightly browned, about 8 to 12 minutes. Transfer to wire racks to cool.

Store cookies in a tin or in a cookie jar at room temperature up to 2 weeks.