Preheat the oven to 225°F. Use a fluted ring cutter to cut the brioche in half circles slightly larger than the rim of the dishes you will cook the eggs in. Line a baking sheet with parchment paper and lay the slices on top in a single layer. Brush the bread with the butter and top with another sheet of parchment. Top with another baking sheet and bake for 20 minutes, or until golden and crispy. Sprinkle with salt, cool, and reserve in an airtight container.