Butter the inside of a 10-inch cast-iron skillet. Spread one side of each slice of challah with the jam and cut each slice in half. Arrange the slices in the skillet crust-side up.
Whisk the milk, heavy cream, vanilla, eggs, 1 tablespoon of the granulated sugar and 1/4 teaspoon salt in a large bowl until well combined. Pour the mixture over the challah, making sure they are covered evenly. Cover with foil and refrigerate for 4 hours and up to overnight.
Preheat the oven to 350 degrees F.
Uncover the skillet and sprinkle with half of the raspberries and the remaining 1 tablespoon granulated sugar. Bake until puffed and lightly golden, about 40 minutes. Dust with confectioners' sugar and top with the remaining raspberries.