Whisk eggs, milk, sugar, vanilla, cinnamon and salt in a wide shallow bowl until smooth with no streaks of egg white. If holding finished slices briefly, heat oven to 200°F (95°C) and set a rack over a baking sheet.

Heat a large skillet over medium heat. Add a portion of the butter and swirl to coat. It should foam without turning dark immediately. Lower the heat if it browns too quickly.

Dip only the slices that fit in the skillet, about 3–5 seconds per side for soft brioche. Lift gently and let excess custard drip off. Place straight in the skillet without crowding. Rewhisk custard between batches.

Cook about 2–4 minutes per side, adjusting heat so both sides brown while the center sets. The center should be tender with no liquid egg custard and reach 160°F (71°C). If the outside darkens early, lower the heat and continue gently. Add the remaining butter between batches as needed.

Serve promptly with optional berries or syrup. Keep finished slices in a single layer on the rack in the warm oven only while completing the batch; avoid stacking. Refrigerate leftovers within 2 hours and use within 3–4 days, reheating to 165°F (74°C).