1Heat oven to 400°F. In medium bowl, cut 1/3 cup butter into flour, using pastry blender (or pulling 2 table knives through ingredients in opposite directions), until mixture is crumbly. Stir in egg until dough forms. Press firmly and evenly against bottom and side of ungreased 9-inch tart pan with removable bottom. Bake 12 to 15 minutes or until golden brown; cool on cooling rack.
2Reduce oven temperature to 350°F. In 2-quart saucepan, melt 1 tablespoon butter over low heat. Stir in milk and chocolate chips. Cook over low heat, stirring occasionally, until chocolate is melted. Stir in walnuts and vanilla. Spread in baked crust.
3Bake about 25 minutes or until edge is set but chocolate appears moist in center. Cool completely in pan on cooling rack, about 1 hour. To serve, top each slice with whipped cream.