Cut a 1-inch slice from bottom of 8 squash, allowing squash to sit flat. Remove stems from tops. Scoop out and discard seeds. Sprinkle squash with salt and pepper, and place, cut side up, in a lightly greased 15- x 10-inch jellyroll pan.
Bake at 350° for 30 minutes or until tender. Cool slightly. Scoop out squash pulp, leaving 1/4-inch-thick shells. Reserve pulp; place shells in plastic bags. Seal and chill 8 hours.
Peel remaining acorn squash; remove seeds, and chop pulp.
Remove and discard green tops from leeks, and discard. Cut white portions in half lengthwise, and cut into 1/2-inch slices.
Melt 2 tablespoons butter in a large Dutch oven over medium or medium-high heat; add chopped squash, leeks, garlic, and 2 tablespoons thyme. Sauté 10 minutes. Add broth, and cook 30 minutes or until squash is tender. Stir in reserved baked squash pulp. Cool slightly.
Process mixture, in batches, in a food processor or blender until smooth. Return to Dutch oven; cover and chill 8 hours.
Stir half-and-half into soup; cook until heated. (Do not boil.)
Melt remaining 2 tablespoons butter in a large skillet. Add squash shells, cut sides down, and cook 2 minutes or until edges are browned. Serve soup in shells, and garnish, if desired.