In a bowl, cream butter and sugar. Beat in the egg, yolk and vanilla. Combine flour, baking powder and salt; gradually add to the creamed mixture and mix well. Form into a long log; cut into eight equal pieces. Shape each into a ball; wrap in plastic. Refrigerate for 1 hour.
Roll balls in additional sugar; place between two sheets of waxed paper. Roll each into a 6-in. circle. Remove top sheet of waxed paper; flip the circles onto ungreased baking sheets. Remove waxed paper; prick dough with a fork. Bake at 350° for 10-12 minutes or until lightly browned. Carefully loosen cookies and cool on paper towels.
For frosting, melt chocolate chips with half-and-half in a heavy saucepan, stirring occasionally. Cool. Combine whipped cream and vanilla; fold into chocolate mixture. Layer cookies, spreading 1/4 cup frosting between each layer. Spread remaining frosting over sides and top. Decorate with chocolate sprinkles. Refrigerate overnight before cutting.
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