Preheat oven to 300°F and remove pumpernickel slices from freezer. Let thaw for 5 minutes. Mix butter with olive oil in a small bowl. Use a cat cookie cutter to cut a shape from each partially frozen slice of pumpernickel. Lightly brush cutouts with butter mixture and arrange on a cookie sheet. Bake until crisp, about 15 minutes. Cool croutons in pan on a rack.