In a medium saucepan, combine whole milk, heavy cream, granulated sugar (for soup), brown sugar, vanilla extract, ground cinnamon (for soup), ground nutmeg, and salt. Heat over medium heat, stirring occasionally, until the mixture is hot and steaming, but not boiling.

In a small bowl, lightly whisk the egg yolks. Gradually pour about 1/2 cup of the hot milk mixture into the egg yolks, whisking constantly to temper them.

Pour the tempered egg yolk mixture back into the saucepan with the remaining hot milk mixture. Add the torn stale brioche bread pieces to the saucepan. Continue to cook over medium-low heat, stirring constantly with a whisk or rubber spatula, until the soup base thickens slightly, coating the back of a spoon (about 8-10 minutes). Do not let it boil.

Remove the saucepan from the heat. Carefully transfer the soup mixture to a blender. Blend on high until completely smooth and creamy. Alternatively, use an immersion blender directly in the saucepan. Strain the soup through a fine-mesh sieve into a clean bowl or back into the saucepan to remove any remaining bread particles or egg solids, ensuring a silky texture.

If serving chilled, cover the soup and refrigerate for at least 2 hours, or until thoroughly chilled. For a warm soup, keep it warm over low heat or reheat gently before serving.

While the soup is cooking or chilling, prepare the croutons. In a small bowl, toss the fresh brioche cubes with melted butter until evenly coated.

Heat a small skillet over medium heat. Add the buttered brioche cubes and cook, stirring frequently, until golden brown and crisp on all sides (about 5-7 minutes).

In a separate small bowl, combine 1 tbsp granulated sugar and 1/2 tsp ground cinnamon. Remove the toasted croutons from the skillet and immediately toss them in the cinnamon sugar mixture until evenly coated. Let cool slightly.

Ladle the warm or chilled Cinnamon Sugar Donut Bread Dessert Soup into individual serving bowls. Garnish generously with the cinnamon sugar croutons. Serve immediately.