In a large bowl, combine flour and salt; cut in butter until crumbly. Whisk ¾ cup milk and egg yolk; gradually add to flour mixture, tossing with a fork until dough forms a ball. Add additional milk, 1 tablespoon at a time, if necessary.

Divide dough into 2 portions so that 1 is slightly larger than the other; cover each and refrigerate 1 hour or until easy to handle.

Preheat oven to 375°. Roll out larger portion of dough between 2 large sheets of lightly floured waxed paper into an 18x13-in. rectangle. Transfer to an ungreased 15x10x1-in. baking pan. Press onto the bottom and up sides of pan; trim crust to edges of pan.

In a large bowl, combine sugar and cornstarch. Add raspberries and rhubarb; toss to coat. Spoon into crust.

Roll out remaining dough; place over filling. Fold bottom crust over edges of top crust; seal with a fork. Prick top with a fork.

Bake until golden brown, 45-55 minutes. Cool completely on a wire rack.

For icing, combine confectioners' sugar, vanilla and enough milk to achieve a drizzling consistency; drizzle over pie. Cut pie into squares.