2 cups heavy cream
3 tablespoons unsweetened Dutch-process cocoa powder
5 ounces bittersweet or semisweet chocolate
1 cup whole milk
¾ cup granulated sugar
Pinch of salt
5 large egg yolks
½ teaspoon vanilla extract
20 regular-size peanut butter cups, chopped
Peanut Butter Swirl:
½ cup peanut butter
½ cup heavy whipping cream
¼ cup light brown sugar
2 tablespoons light corn syrup
1. Warm 1 cup of the cream with the cocoa powder in a medium saucepan, whisking to thoroughly blend the cocoa. Bring to a boil, then reduce the heat and simmer at a very low boil for 30 seconds, whisking constantly. Remove from the heat and add the chopped chocolate, stirring until smooth. Then stir in the remaining 1 cup cream. Pour the mixture into a large bowl, scraping the saucepan as thoroughly as possible, and set a mesh strainer on top of the bowl.
2. Warm the milk, sugar, and salt in the same saucepan. In a separate medium bowl, whisk together the egg yolk. Slowly pour the warm milk into the egg yolks, whisking constantly, then scrape the warmed egg yolks back into the saucepan.
3. Stir the mixture constantly over the medium heat with a heatproof spatula, scraping the bottom as you stir, until the mixture thickens and coats the spatula (170°F on an instant-read thermometer). Pour the custard through the strainer and stir it into the chocolate mixture until smooth, then stir in the vanilla. Stir until cool over an ice bath.
4. While the custard cools, make the peanut butter swirl. Combine the peanut butter, whipping cream, corn syrup, and the brown sugar in a small saucepan saucepan. Heat over low heat, stirring, until smooth and sugar is dissolved. Transfer to an airtight container.
5. Chill both the custard and the peanut butter swirl thoroughly, at least 8 hours.
6. Freeze the custard mixture in your ice cream maker according to the manufacturer’s instructions. (If the cold mixture is too thick to pour into your machine, whisk it vigorously to thin it out.) During the last two minutes, add in the chopped peanut butter cups. When packing the ice cream into its container, alternate layers of the ice cream and peanut butter swirl, ending with the ice cream.