In a large mixing bowl, whisk together the eggs, milk, sugar, cinnamon, and salt until the sugar is mostly dissolved. Add the bread cubes and gently press them down with a spatula until they absorb the liquid — don't over-mix or the bread will fall apart. Let this mixture rest for at least 20 minutes, or ideally overnight in the fridge, so the custard fully penetrates.

Pour the soaked bread mixture into the slow cooker, spreading it evenly into a single layer. Cover and cook on low for 6 to 7 hours until the center is set and a knife inserted comes out clean. Do not stir during cooking, as this will break the custard structure.

While the casserole is cooking, whisk the heavy cream, softened butter, and vanilla extract together in a small bowl until smooth and the butter is fully melted. Keep this drizzle refrigerated until serving so it stays thick enough to pour.

Let the casserole rest for 10 minutes after cooking to firm up slightly. Spoon into bowls or serve directly from the slow cooker insert, drizzling generously with the cold vanilla cream right before each person eats so the contrast of warm custard and cool cream shines.