Set oven to 450°F. Line a baking sheet with parchment paper.
In a bowl, combine pumpkin, cheddar, salt, chili powder, and paprika. Stir well.
Butter one side of each bread slice.
Place 4 slices, buttered side down, on the sheet. Spread half the filling, top with arugula, add remaining filling, and close with the other slices, buttered side up.
Bake 6–8 minutes until golden. Flip and bake 6 more minutes until crisp and hot.