Preheat the oven to 325 degrees. Line a baking sheet with parchment to prevent sticking and for easier cleanup.
Pour the melted butter into a bowl. Whisk together the cinnamon and sugar in a wide, shallow dish.
Stack the slices of bread and cut them diagonally into quarters to get 24 triangles.
Use a pastry brush to generously brush the butter onto both sides of one triangle of bread. Then, dip the bread into the cinnamon sugar to coat on both sides, being careful not to make it too thick. Lay it on the prepared baking sheet, then repeat with the remaining bread triangles.
Bake for about 25 minutes, until lightly browned. Allow to cool to room temperature - they'll crisp up as they sit. Once cooled, store in an air tight container at room temperature for up to two weeks.
Now here's the really strange and difficult part: These are not the kind of cookie that tastes better straight from the oven. In fact, they are much, much better once they've aged a day or two and the flavors have time to meld.