Place the raisins in a bowl and pour the hot water over them. Let them soak for at least 30 minutes, or even longer, until they're plump. Drain them well before using.

In a saucepan, heat the milk and half of the sugar over medium heat until it's just simmering (small bubbles around the edges).

In a separate bowl, whisk together the egg yolks, remaining sugar, and cornstarch until smooth.

Slowly pour a small amount of the hot milk into the egg yolk mixture, whisking constantly (this is called tempering, and it prevents the eggs from scrambling).

Pour the tempered egg yolk mixture back into the saucepan with the remaining milk.

Cook over medium heat, whisking constantly, until the cream thickens enough to coat the back of a spoon. This usually takes a few minutes.

Remove from heat and stir in the vanilla extract.

Pour the pastry cream into a bowl, cover it directly with plastic wrap (to prevent a skin from forming), and chill it in the refrigerator until completely cold.

In a large bowl, whisk together the flour, sugar, and salt.

Add the cubed cold butter to the flour mixture. Use your fingertips, a pastry blender, or two forks to cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter.

Gradually add the cold milk (or water), mixing until a shaggy dough forms. Don't overmix!

Turn the dough out onto a lightly floured surface and gently knead it a few times just to bring it together.

Form the dough into a rectangle, wrap it tightly in plastic wrap, and chill it in the refrigerator for at least 1 hour, or preferably 2 hours.

Place the cold butter between two sheets of parchment paper.

Using a rolling pin, pound the butter into a flat, rectangular shape (about 6x8 inches). Keep it relatively even in thickness.

Wrap the butter block in plastic wrap and chill it in the refrigerator while the dough is chilling. The butter and dough should be at similar temperatures.

Lightly flour your work surface.

Roll the chilled dough into a rectangle that's about twice the size of the butter block (around 12x16 inches).

Place the chilled butter block in the center of the dough.

Fold the dough over the butter like a letter, enclosing the butter completely. Pinch the edges to seal.

Gently roll the dough into a long rectangle (about 18-20 inches long). Be careful not to press too hard, or the butter will squeeze out.

Fold the dough into thirds, like a letter again. This is your first "turn."

Wrap the dough in plastic wrap and chill it for at least 30 minutes.

Repeat the rolling and folding process (Step 5) two more times, chilling the dough for 30 minutes after each turn. So, you'll roll, fold, chill; roll, fold, chill; roll, fold, chill.

After the final chill, roll the dough into a large rectangle (about 12x18 inches) and about 1/4 inch thick.

Spread the chilled pastry cream evenly over the dough, leaving a small border around the edges.

Sprinkle the drained, plumped raisins evenly over the pastry cream.

Starting from one of the long sides, tightly roll the dough into a log.

Using a sharp knife, cut the log into 1-inch thick slices.

Place the sliced Pain aux Raisins on a baking sheet lined with parchment paper, leaving some space between them.

Lightly cover them with plastic wrap and let them proof (rise) in a warm place for about 1-2 hours, or until they're puffy and almost doubled in size.

Preheat your oven to 375°F (190°C).

Whisk together the egg and milk (or water) for the egg wash.

Gently brush the tops of the proofed Pain aux Raisins with the egg wash.

Bake for 18-22 minutes, or until they're golden brown and cooked through.

Let the Pain aux Raisins cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely. They're best enjoyed slightly warm or at room temperature.