Preparation

Make the streusel

• Put the flour, oats, sugar, cinnamon, and salt in a medium bowl, and stir until evenly combined. Add the butter and syrup, and stir until moist clumps form and there are no dusty bits at the bottom of the bowl.

Make the muffins

• Generously coat a standard 12-cup muffin tin with 1 Tbs. of the butter.
• In a small saucepan or microwave-safe bowl, melt the remaining 4 Tbs. butter, then transfer to a large bowl. Add the milk, sugar, eggs, vanilla, cinnamon, and salt, and whisk until combined. Fold the bread into the egg mixture until evenly moistened. Divide the soaked bread among the wells, then gently press the top of each to flatten slightly. Sprinkle the streusel evenly over the tops of the muffins, and gently press it down with your hands. Cover the pan with plastic wrap, and refrigerate at least 2 hours or overnight.
• Position a rack in the center of the oven, and heat the oven to 350°F. Uncover the muffins, and bake until light golden brown and a tester inserted in the center comes out clean, 20 to 25 minutes. Let the muffins cool in the pan for 5 minutes. Run a thin knife around each muffin, then remove from the pan and transfer to a wire rack. Let cool 5 minutes before serving warm.