Heat oven to 350°F. Butter and flour a large baking sheet. Combine dry ingredients. Cream butter and sugar together 5 minutes till light and fluffy. Add eggs and beat till well combined. Stir in flour mixture to form a stiff dough. Stir in walnuts and chocolate chips. Divide the dough in half. On prepared baking sheet, shape into two slightly flattened logs (about 12x2 inches). Sprinkle with powdered sugar. Bake 35 minutes till slightly firm to the touch. Cool logs on baking sheet for 5 minutes, then remove to a cutting board. Cut logs diagonally into about 3/4 inch thick slices with serrated bread knife. Place back on the baking sheet, cut sides down. Return to oven and bake for about another 7-10 minutes, turning over halfway through baking time. Cool on rack.