Make brioche dough the day before making pastry and chill.
Just before making pastry cream, soak raisins in boiling-hot water until softened, about 10 minutes. Drain, pressing out excess liquid, and cool to room temperature.
Make pastry cream: Bring milk to a simmer in a 1 1/2-quart heavy saucepan. Whisk together yolks, sugar, and cornstarch in a bowl and gradually whisk in hot milk. Return mixture to pan and cook over moderately low heat, stirring with a wooden spoon, until mixture begins to boil. Simmer, stirring, until thickened and smooth, about 3 minutes.
Transfer to a clean bowl and stir in vanilla and butter. Cover surface with plastic wrap and cool to room temperature.
Make pastries: Roll out brioche dough on a well-floured surface into an 18- by 11-inch rectangle with a short side toward you. Spread pastry cream evenly over dough, leaving a 1/2-inch border at top edge. Sprinkle raisins evenly over cream. Roll up dough, starting from bottom, to make a log 11 inches long and about 3 1/2 inches in diameter. Moisten top edge with water and press to seal closed.
Transfer to a cutting board or baking sheet and cover loosely with plastic wrap.
Chill until firm, about 1 hour.
Cut chilled log into 11 (1-inch-thick) rounds and arrange about 2 inches apart on 2 buttered baking sheets.
Let pastries rise in a warm place, uncovered, 1 hour. (They will increase slightly in size and feel very tender to the touch.)
While pastries are rising, preheat oven to 425°F.
Bake in batches in middle of oven until tops are golden brown, 12 to 15 minutes. Transfer pastries to a rack.
Simmer preserves and water, stirring, 1 minute. Pour through a sieve into a bowl, pressing on solids. Brush glaze onto pastries.
Cooks' notes: ·Uncut log can be chilled overnight if desired. ·Pains aux raisins can be frozen 1 month, thawed, and reheated in a 350°F oven. However, the pastries really are best when eaten the day they are made.