Arrange a rack in the middle of the oven and heat to 300°F.
Fit a wire rack inside a rimmed baking sheet.
Slice your challah loaf into 1 inch slices. Place the bread in a single layer on the rack. Bake, flipping halfway through, until dry to the touch on both sides, about 15 minutes total. Remove from oven and let cool 5 minutes.
While the bread is making, make your tres leches custard. Mix the condensed milk, evaporated milk, salt, grand marnier and vanilla in a large bowl. Once it is well combined, pour 1 and a half cup of the tres leches syrup in a cup and reserve for later.
In the remaining tres leched syrup, mix in your eggs and cinnamon. Whisk until fully combined (no streaks of egg remaining).
Begin soaking your challah slices. Completely submerge the slices in the custard until drenched but not falling apart, 1 to 2 minutes per side. When removing the slice, lightly tap the bread against the side of the bowl to remove any excess custard.
Set a large skillet to medium-low heat and melt 1 tablespoon of butter. Add 2-3 soaked challas slices Cook until the bottoms are golden-brown, about 4 minutes. Flip and cook until the second side is browned, about 3 minutes more. Add additional butter to the pan in between batches, swirling as it melts.
Arrange your cooked slices in a casserole dish and pour over the top the reserved tres leches syrup.
Serve these warm, garnish with whipped cream and berries.