In a large mixing bowl cut butter into flour until mixture resembles coarse crumbs. Stir in sour cream and cheddar cheese. Divide mixture in half; wrap and refrigerate for 1 hour or until dough is firm (or, if desired, refrigerate overnight).
On a floured surface, roll out one portion of dough to 1/16-inch thickness. Using a 2-inch floured biscuit cutter, cut into 48 rounds. Using a round hors d'oeurvre cutter, cut a 1-inch circle out of the center of the center of 32 of the dough rounds. Place remaining 16 rounds on an ungreased baking sheet; brush with milk. Top each with 2 dough rounds with centers removed; making 3 layers in all. Brush with additional milk between layers. Repeat with remaining dough and reroll center cutouts to make a total of 32 layered rounds. Bake in a 350 degree F oven for 22 to 25 minutes or until golden brown. Remove and cool on wire racks.
Meanwhile, for the filling, set aside 32 shrimp for garnish. Chop remaining shrimp. Beat cream cheese, the 1/4 cup milk, lemon juice, Worcestershire, dillweed, and garlic powder; stir in the chopped shrimp.
Spoon fillling into pastry rounds. Top with reserved shrimp. Garnish with fresh herb sprigs. Makes 32 appetizers.