Cream butter and sugar until light and fluffy. Beat in egg and vanilla. Combine flour, cornstarch, baking powder and salt. Fold dry ingredients into creamed mixture; then mix until well blended. (Chill dough 20-30 minutes if necessary for ease of handling.)
Form dough into two 10-inch long logs on sheets of parchment paper or waxed paper. Position each log in the center of the parchment paper. Pour half of the chopped walnuts in a line parallel with each log of dough; roll logs in nuts to thoroughly cover the outside of each log. Wrap each log in the parchment paper and chill for 2 or 3 hours.
Preheat oven to 350°F.
Slice the log into ¼-inch slices and put about 1-inch apart on an ungreased cookie sheet.
Bake 12-14 minutes or until the edges are lightly browned.
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