Preheat the oven to 375 degrees
Grease a 9 x 13 x 3 baking dish with 1 tbsp butter
Spread the croissant cubes evenly in the dish and top with berries
Use a mixer and beat the 12 tbsp butter with granulated sugar until creamy. This takes a couple minutes to do.
Beat in the eggs, one at a time, beating well after each addition.
Beat in the cream, buttermilk, vanilla and salt.
Pour over the croissants and berries. Sprinkle with coarse sugar
Bake until a knife inserted near the center comes out clean, about 35 minutes.
Serve warm. Leftovers last about 2 days.