Combine 1/2 cup sugar, 2 tablespoons cornstarch and teaspoon salt in a medium bowl; slowly whisk in 1 cup milk, then whisk in 1 large egg. Meanwhile, bring another 1 cup milk to a boil in a medium saucepan over medium heat. Remove from the heat and whisk in the sugar mixture. Return to medium-low heat and simmer, whisking, until thick and creamy, 5 to 7 minutes. Remove from the heat and whisk in 1 tablespoon each vanilla and unsalted butter until smooth. Add stir-ins (see below). Divide among ramekins. Cover with plastic wrap and refrigerate until set, about 2 hours.
Gingerbread cookie:Stir in 1/4 cup cookie butter and layer with crumbled gingersnaps
Double chocolate chip:Stir in 1/2 cup milk chocolate chips and layer with crumbled chocolate chip cookies
Chocolate-Mint: Stir in 1/2 cup mint baking chips and layer with crumbled chocolate sandwich cookies
Caramel crunch: Stir in 1/2 cup dulce de leche and layer with broken pretzels
Pumpkin pie: Stir in 1/2 cup pumpkin puree and layer with crushed graham crackers
Chocolate peanut butter: Stir in 1/2 cup semisweet chocolate chips and layer with crumbled peanut butter cookies