Semifreddo is a partly frozen blend of whipped cream, egg whites, and sugar; it's cool, seductive, and melt-in-your-mouth soft. We've given it crunch with ground chocolate and macadamia nuts.
Preheat oven to 350 degrees. Line a baking sheet with parchment paper; set aside. Bring butter, sugar, salt, and 1/2 cup water to a boil in a small saucepan over medium-high heat. Remove from heat; stir in flour. Return to medium-high heat; stir with a wooden spoon until batter pulls away from sides of pan, about 4 minutes.
Transfer batter to the bowl of an electric mixer fitted with the paddle attachment. Mix on low speed until slightly cooled, about 2 minutes. With mixer on medium speed, add eggs 1 at a time, mixing well after each addition. Test batter for doneness by dabbing it with your finger; as you pull away, a sticky thread should form. If not, add the egg white 1 teaspoon at a time.
Fit a pastry bag with a 1/2-inch round tip, and fill with batter. Pipe 1 1/2-inch diameter rounds 2 inches apart on lined sheet. Gently smooth peaked tops with a moistened fingertip.
Bake until golden brown, 28 to 30 minutes. Let cool completely on a wire rack. If not using immediately, store at room temperature in an airtight container up to 3 days, or freeze up to 1 month.
To assemble profiteroles, halve them horizontally, and fill with a 1 1/2-ounce scoop (3 tablespoons) of semifreddo. Top profiteroles with chocolate sauce.