To prepare shells, preheat oven to 350°F Spray 24 nonstick mini-muffin cups with cooking spray.
Pulse the flour, cornstarch, sugar, and salt in a food processor. Add cream cheese, butter, and milk. Pulse until mixture forms into fine, damp crumbs. Press into a ball and knead gently a few times.
Divide the dough into 4 pieces; then divide each one into 6 tablespoon size pieces. Gently press each piece into the bottom and up the sides of the prepared muffin cups. Make a few holes in the bottom of each cup with a fork. Lightly brush the inside of each cup with the egg yolk. Bake until golden along the edges (about 20 min). Cool completely.
To assemble, spread 1 tsp pudding into each cooked shell. Top with berries. Just before serving, sift a light dusting of confectioners sugar over the tartlets.