Brush any loose powdered sugar off the Mallorca rolls. Using a serrated knife, split each roll horizontally into a top and bottom half, making 8 pieces. Aim for halves about 3/4–1 inch thick.
In a wide shallow dish, whisk the eggs with the cinnamon and salt until well combined. Slowly whisk in the milk and vanilla until no obvious streaks of egg remain.
Heat a large nonstick skillet or griddle over medium-low heat. Add about 1 tablespoon of butter for the first batch and let it melt without browning heavily.
Dip only as many bread halves as will fit in the pan. Start with about 5 seconds per side for soft bread, or up to 10 seconds per side for firmer, day-old bread. Coat the cut surfaces thoroughly, but lift the pieces out before they become floppy. Allow excess custard to drip back into the dish.
Place the dipped halves in the skillet, cut-side down, leaving space between them. Cook for 3–4 minutes, until golden underneath.
Turn gently with a wide spatula and cook for another 2–3 minutes, adjusting the heat as needed. The bread should feel lightly springy, with a cooked, tender center and no wet egg mixture inside. For a temperature check, the center should reach 160°F (71°C).
Transfer the cooked French toast to plates. Carefully wipe away any dark butter residue from the pan, add the remaining butter, and repeat with the remaining bread. Stir the custard briefly before dipping the next batch.
Dust with powdered sugar just before serving. Serve warm, with maple syrup on the side if desired.