Make space. Set out a cutting board, a rolling pin, and two shallow bowls. One will hold the egg mixture, the other the cinnamon sugar. Line a plate with paper towel for the cooked roll-ups.
Prep the bread. Trim crusts from each slice. Use a rolling pin to gently flatten the bread to about half its original thickness. Don’t press it into paper, just enough to make it flexible and easy to roll.
Add the filling. Spread a very thin layer of your chosen filling on each slice, stopping about 1/4 inch from the edges. Less is more here. Think a crayon line of jam or a whisper of cream cheese. If you’re adding fruit, line up a few small pieces along one short edge.
Roll it up. Starting from the edge with fruit (if using), roll each slice tightly into a log. Place seam-side down so they don’t unwind.
Mix the custard. In a shallow bowl, whisk together the eggs, milk, vanilla, and a pinch of salt until smooth.
Make cinnamon sugar. In the other shallow bowl, combine the granulated sugar and cinnamon. Adjust to taste. If you like a stronger cinnamon note, use the full 1 1/2 teaspoons.
Heat the pan. Set a large nonstick skillet over medium heat. Add 1 tablespoon of butter and let it melt and foam slightly. You want a gentle sizzle, not smoke.
Dip and cook. Working in batches, quickly dip each roll-up in the egg mixture, turning to coat, then lift and let excess drip off. Place seam-side down in the skillet. Cook, turning every 1 to 2 minutes, until all sides are golden brown and lightly crisp, about 6 to 8 minutes total. Add small pats of butter between batches as needed.
Coat with cinnamon sugar. As soon as each batch comes off the heat, roll the hot roll-ups in the cinnamon sugar to coat. The warmth helps the sugar cling.
Serve. Pile them on a plate and serve right away with maple syrup, fresh berries, or just as they are. They’re best hot, when the edges still have that gentle crunch.