Almond, Chocolate, and Pistachio Spumoni

Preparation Bring milk to a simmer in a 4-quart heavy saucepan over medium heat. Whisk together yolks, sugar, cornstarch, and a pinch of salt in a bowl. Whisk in milk, then transfer mixture to saucepan. Bring to a boil over medium heat, whisking constantly, then boil, whisking constantly, 2 minutes. Immediately strain through a fine-mesh sieve into a clean bowl. Divide custard among 3 bowls. To make almond base, whisk 1/2 teaspoon vanilla into 1 bowl; for pistachio base, whisk almond extract and food coloring into another bowl; for chocolate base, whisk cocoa and remaining 1/2 teaspoon vanilla into third bowl. Cool to room temperature, stirring occasionally. Chill custards, surfaces covered with parchment paper (to prevent a skin from forming), until cold, at least 6 hours. Freeze each custard in ice cream maker, 1 batch at a time (white, then green, then brown). Fold almonds into vanilla custard, then fold pistachios into green custard. Transfer to a shallow 2-quart container or 3 smaller containers and put in freezer to firm up, about 4 hours. Cooks' notes:•Custards can be chilled up to 1 day. •Spumoni can be made 3 days ahead.