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1
Preheat the oven to 350 degrees F and grease a 9-by-13-inch baking dish; set aside.
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2
Spread the bread cubes on a rimmed baking sheet and bake until dry and lightly golden, stirring often, about 20 minutes.
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3
Let cool completely.
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4
Warm 1/2 cup of the heavy cream and set aside.
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5
Combine 1 cup of the sugar, the cream of tartar, and 1/4 cup water in a medium saucepan.
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6
Using your wet hand, wash down any sugar crystals from the sides of the pan.
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7
Cook over medium heat, gently swirling the pan but not stirring, until the sugar melts and turns amber, about 10 minutes.
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8
Remove from the heat and carefully whisk in the butter, the warmed heavy cream, and 3/4 teaspoon of the kosher salt, being careful of any splattering.
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9
Transfer to a bowl and set aside.
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10
Whisk the eggs, the remaining 3/4 cup of sugar, and remaining 1/2 teaspoon of kosher salt in a large bowl, then whisk in the milk, the remaining 1 cup of heavy cream, and vanilla until smooth.
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11
Fold in the toasted bread and the banana slices.
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12
Transfer half of the mixture to the prepared baking dish and spread in an even layer.
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13
Drizzle with 1/2 cup of the caramel and top with the remaining bread mixture.
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14
Bake until the strata is golden brown, the bread springs back when gently poked with a finger, and no liquid oozes into the center, about 1 hour.
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15
Just before serving, drizzle with the rewarmed caramel and lightly sprinkle with sea salt.
