2 cups whole milk
1 and 1/2 cups sugar
2 cups creamy peanut butter (not natural)
4 ounces semisweet chocolate chips (3/4 cup)
2 cups heavy cream
1 teaspoon vanilla extract
Line a 5-by-10-inch jumbo loaf pan with parchment paper, leaving an overhang on all sides. In a medium saucepan, combine milk and 1 cup sugar. Bring to a simmer over medium-high, stirring constantly, until sugar dissolves. Remove from heat, add peanut butter, and whisk until smooth. Transfer to a large bowl and let cool to room temperature. In a small saucepan, bring 1/2 cup sugar and 1/2 cup water to a boil over medium-high, stirring occasionally until sugar dissolves. Remove from heat, add chocolate, and stir until chocolate melts and mixture is smooth. Transfer to a small bowl and let cool to room temperature.
In a large bowl, using an electric mixer, whisk cream and vanilla until soft peaks form. Add half the peanut butter mixture and whisk until combined. With a rubber spatula, add half the remaining peanut butter mixture and gently fold 3 times (mixture will be rippled).
Transfer half the cream mixture to a loaf pan. Top with half the peanut butter mixture and 1/4 cup chocolate mixture. Top with remaining cream mixture, then drizzle with remaining peanut butter mixture and 1/4 cup chocolate mixture.
With a wooden skewer, swirl mixtures together. Freeze until firm, about 5 hours. Serve with extra chocolate sauce.