In a small bowl, mix the softened butter, brown sugar, cinnamon, and salt until evenly combined. The mixture should be smooth and spreadable rather than runny.

Remove the crusts from the bread slices: Working one slice at a time, flatten each slice with a rolling pin until it is thin and flexible, about 1/8 inch (3 mm) thick. Avoid pressing so hard that the bread tears.

Spread about 1 teaspoon of the cinnamon filling evenly over each flattened slice, leaving a narrow border around the edges. Starting from one short end, roll each slice tightly into a cylinder. Place each roll seam-side down while preparing the remaining pieces.

Whisk the eggs, milk, maple syrup, vanilla, cinnamon, and salt in a shallow bowl until completely blended. The mixture should be smooth with the cinnamon evenly distributed.

Place a 10- to 12-inch nonstick skillet over medium-low heat. Add 1 tablespoon (14 g) of butter and let it melt until foamy but not browned.

Dip each roll-up briefly into the custard, turning to coat all sides. Do not soak them for more than a few seconds, or the flattened bread may become difficult to handle.

Place the coated roll-ups seam-side down in the skillet. Cook for about 1 to 2 minutes, turning gently as each side becomes golden brown. Work in batches if needed so the skillet is not crowded.

Continue turning the roll-ups every 1 to 2 minutes until the bread is golden brown on all sides and the center is hot. Add the remaining butter to the skillet between batches as needed.

The exterior should be golden and lightly crisp, while the bread inside should be tender and the egg custard fully set. If the outside browns too quickly, lower the heat and continue cooking more gently.

Transfer the roll-ups to a serving plate: Dust with optional powdered sugar or serve with maple syrup for dipping.