Place espresso in a small microwave safe bowl. Add liqueur and microwave on HIGH (100%) for 10 to 15 seconds to steep; set aside.
Beat butter and sugar at medium speed with an electric mixer until light and fluffy. Add eggs, one at a time, beating until blended; stir in coffee mixture. Combine flour, baking powder, and salt; add to butter mixture, stirring until blended. Fold in nuts. Divide dough in half.
On a greased baking sheet, shape dough into two 13"x 1 1/2"x 1/2" rectangles, spacing them about 2 inches apart. Bake at 325° for 20 to 25 minutes or until golden. Remove to a wire rack; let cool 5 minutes.
Place biscotti on a cutting board; using a serrated knife, cut biscotti diagonally into 1/2-inch-thick slices. Place slices upright on baking sheet 1/2" apart and bake 10 more minutes. Let cool on rack. Store in a tightly covered container.