Combine flour, brown sugar and nuts in a large bowl; cut in butter until mixture resembles coarse meal. Mix in milk and vanilla with a fork just until blended. Pat onto bottom and up the sides of an 11-in. tart pan; set aside. , For filling, melt chocolate and chips in the top of a double boiler over hot water. Remove from heat and stir in butter. Place in a large bowl and combine with sugar. Add eggs and vanilla; blend well. Gradually add flour, blending well after each addition. Add nuts. Pour over crust. , Bake at 350° for 30-35 minutes, or until center is just set and toothpick comes out clean. Cool on wire rack. , Meanwhile, for frosting, melt chocolate in a small saucepan over low heat. Add milk, butter, yolk and vanilla. Heat, stirring vigorously, until smooth and thick, about 5 minutes. Spread over tart. Garnish with whipped cream and pecans if desired.