Melt butter in a large Dutch oven over medium-high heat.
Add squash, potato, salt, and pepper to pan; sauté 3 minutes. Add leek; sauté 1 minute. Stir in broth; bring to a boil. Reduce heat, and simmer 20 minutes or until potato is tender, stirring occasionally.
Place half of potato mixture in a blender. Remove center piece of blender lid (to allow steam to escape); secure blender lid on blender. Place a clean towel over opening in blender lid (to avoid splatters). Blend until smooth. Pour into a large bowl.
Repeat procedure with remaining potato mixture.
Stir in half-and-half.
Cover and keep warm.
For Gruyere Toasts:
Arrange bread slices in a single layer on a baking sheet; sprinkle evenly with cheese. Broil bread slices 2 minutes or until golden.
Ladle 1 cup soup into each of 8 bowls; top each serving with about 1 teaspoon chives. Serve 2 bread slices with each serving. Garnish with freshly ground black pepper, if desired.