To prep ahead, make the curd up to two days in advance, and chill. Whip the meringue an hour before your party; assemble just before serving.
Prepare Curd: Whisk together first 5 ingredients in a medium saucepan. Cook mixture over medium heat, stirring constantly with a wooden spoon, 10 to 12 minutes or until thickened and bubbles form around edges.
Remove from heat, and add butter, 1 slice at a time, whisking until blended after each addition. Transfer mixture to a medium-size glass bowl, and place plastic wrap directly on warm curd. Chill 4 to 6 hours.
Prepare Meringue: Pour water to a depth of 2 inches into a 2-qt. saucepan. Bring to a boil over medium-high heat. Reduce heat to medium, and let simmer. Beat egg whites and next 2 ingredients in a 2 1/2-qt. bowl at medium speed with an electric mixer until blended. Place bowl over simmering water, and beat at medium speed 9 minutes or until soft peaks form. Remove from heat. Beat at high speed 2 minutes or until stiff peaks form.
Divide chilled lime curd among ice-cream cones.
Spoon meringue into a large zip-top plastic freezer bag. (Do not seal.) Snip 1 corner of bag to make a small hole. Pipe meringue over curd in each ice-cream cone. If desired, brown meringue using a kitchen torch, holding torch 2 inches from meringue and moving torch back and forth.
Key Lime Parfaits: Prepare Key Lime Curd Cones through Step Omit cones. Divide 2 cups graham cracker crumbs among 6 bowls or jars; top with lime curd. Proceed with recipe as directed in Step Makes 6 servings
Note: We tested with Joy Mini Cups ice-cream cones, and recommend using a pro-style kitchen torch ($20 at amazon.com or housewares retailers).