Toast the croissants. Preheat oven to 375 degrees F. Place the chopped croissants on a large baking sheet. Bake for 5 to 8 minutes, tossing once, or until the croissants are golden brown. Set aside to cool. Turn off oven.
Brown the butter. In a medium saucepan, melt the butter over medium heat. Continue cooking, stirring frequently, until the butter foams and turns golden brown with a nutty aroma. This will take 5 to 7 minutes. Remove from heat and pour into a bowl to cool. I place the bowl in the refrigerator for 15 to 20 minutes.
Whisk dry ingredients. In a medium bowl, whisk together the flour, baking soda, and sea salt. Set aside.
Mix wet ingredients. In a large bowl, combine the browned butter, brown sugar, and granulated sugar. Whisk until smooth and glossy. Add the eggs, milk or heavy cream, and vanilla extract. Whisk until fully combined and the mixture is thick and creamy.
Add dry ingredients. Add the dry ingredients to the wet ingredients and stir until just combined, do not overmix.
Add croissants. Chop the remaining croutons into smaller pieces. Add 1 cup of crouton pieces to the cookie dough. Reserve the remaining chopped croissants for rolling and garnish.
Add chocolate. Add the chocolate chunks and stir until just combined. Don’t over mix.
Scoop, roll, and chill. Take about 1/2 cup of the croissant pieces and finely chop them. Place in a bowl. Scoop the dough into cookie balls, about 2 tablespoons per cookie. Roll the balls in the chopped croissant pieces. Place the cookie balls on a sheet or tray lined with parchment paper. Cover and chill for 1 hour or up to 24 hours.
Bake. When ready to bake, preheat the oven to 350 degrees F. Place the chilled cookie dough balls on a large baking sheet lined with parchment paper, about 2 inches apart. Bake for 10 to 12 minutes, or until the edges are set and lightly golden but the centers are still soft.
Garnish. Remove from the oven and sprinkle with flaky sea salt. Use the remaining croissant croutons to garnish the cookies. Break off small pieces and gently press them into the tops of the warm cookies.
Cool. Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to cool completely.