Preheat your oven to 350°F (175°C) and prepare a muffin tin with non-stick spray.

In a large mixing bowl, whisk together milk, granulated sugar, eggs, vanilla extract, and ground cinnamon until smooth.

Gently fold in the cubed stale bread or croissants, ensuring each piece is well-coated.

Divide the bread mixture evenly among the muffin cups, filling each about ⅓ full.

In a separate bowl, combine cold unsalted butter, brown sugar, all-purpose flour, ground cinnamon, and a pinch of salt until crumbly.

Sprinkle the prepared streusel topping over each filled muffin cup.

Bake in the oven for 25-30 minutes, until golden brown and firm.

Allow the muffins to cool for about 5 minutes before transferring to a wire rack.