Preheat the oven to 375°F.
Mix sugar and ground cinnamon in a small bowl.
Arrange bread slices on an ungreased baking sheet.
Brush bread slices with melted butter and sprinkle with the sugar-cinnamon mixture.
Bake the bread slices until lightly browned, about 5 to 7 minutes.
Remove from oven and preheat the broiler.
Mix mashed sweet potatoes, chopped pecans, brown sugar, and melted butter together.
Spread the sweet potato mixture evenly over the toasted bread slices.
Top each with halved miniature marshmallows, pressing lightly.
Broil 3 to 4 inches from heat for 1 to 2 minutes, until marshmallows are lightly toasted.