Drying out the challah lets it absorb the custard without going to mush. You can also use brioche.
Ingredients
1 1-pound loaf challah, sliced 1" thick
Unsalted butter, room temperature (for baking dish)
4 large eggs
4 large egg yolks
2 cups heavy cream
2 cups whole milk
1/2 cup sugar
1 teaspoon vanilla extract
1/2 teaspoons ground cinnamon
1/2 teaspoons ground nutmeg
1/2 teaspoons kosher salt
1/2 cup pecans
2 tablespoons chilled unsalted butter, cut into pieces
2 tablespoons light brown sugar
1/2 teaspoons kosher salt
Blackberry Syrup
Directions
Spread out bread on a rimmed baking sheet; let stand overnight.
Butter a 13x9" baking dish. Cut bread so slices are similar in size. Arrange, overlapping, in rows in prepared dish.
Whisk eggs, egg yolks, cream, milk, sugar, vanilla, cinnamon, nutmeg, and salt in a medium bowl. Pour over bread, pressing bread to help it soak up custard. Cover and chill at least 2 hours.
DO AHEAD: Bread can be soaked 1 day ahead. Keep chilled.
Preheat oven to 375°F. Pulse pecans, butter, brown sugar, and salt in a food processor until nuts are coarsely chopped.
Scatter pecan crumble over soaked bread. Place dish on a rimmed baking sheet and cover tightly with foil. Bake until warmed through (a knife inserted into the center should feel warm to the touch), 25–30 minutes. Remove foil and bake until deeply browned, 35–40 minutes longer. Let cool slightly before serving with Blackberry Syrup.
DO AHEAD: Pecan mixture can be made 1 day ahead. Cover and chill.
