Preheat the oven to 325 degrees F.
Combine warmed milk and melted butter with the sugar, eggs, and vanilla in a large mixing bowl. Whisk until incorporated and smooth.
Line a large baking dish with two-thirds of the challah slices. Sprinkle with half the chocolate chips and half the raisins. Top with remaining challah slices, layering them one on top of the other, and then remaining chocolate chips and raisins, making sure the chips and raisins get inside the layers created by the challah slices.
Pour milk mixture slowly over the top of everything and let stand until the bread has absorbed almost all of the liquid, 5 to 10 minutes.
Wrap the baking dish tightly with aluminum foil and bake for 50 minutes. Remove the aluminum foil and cook 10 to 15 minutes longer until set in the middle and lightly browned on top.
Remove from oven and let cool before serving. Serve with the Chocolate Ganache drizzled over top.
1 cup heavy whipping cream
1 cup semisweet chocolate chips
In a small saucepan, heat heavy cream to just below a simmer.
Place the chocolate chips in a medium heat-proof bowl and pour the hot cream over the chocolate chips.
Cover tightly with plastic wrap and let sit for 5 minutes.
Remove plastic wrap and whisk vigorously until ganache becomes a uniform smooth chocolate sauce. Serve while warm.
Cook's Note: To make in advance, let cool, cover tightly and keep refrigerated until ready to use. Then microwave on low heat for 20 to 30 second intervals or heat over a double boiler.