Preheat your oven to 375 degrees Fahrenheit.
Grease a 9 x 13 x 3-inch baking dish with 1 tablespoon of butter.
Spread cubed croissants evenly in the greased baking dish.
Top with the blueberries, ensuring they are distributed evenly.
In a large bowl, beat the softened butter and granulated sugar with a mixer on medium speed until creamy.
Add the eggs one at a time, mixing well after each addition.
Pour in the heavy cream, buttermilk, and vanilla extract.
Mix in the salt until everything is well combined.
Pour the creamy mixture over the croissant and blueberry base, ensuring all pieces are soaked.
Let it sit for about 10 minutes to allow the bread to absorb the mixture.
Bake in the preheated oven for 1 hour or until the top is golden brown and the center is set.