Heat milk in a medium saucepan over medium heat until hot. Meanwhile, whisk sugar and eggs in a large bowl or bowl of a stand mixer 5 minutes until mixture is thickened and a pale yellow.

Pour milk into sugar-egg mixture. Whisk to combine, then pour mixture back into saucepan. Heat over medium-low heat, stirring constantly, 5 minutes until mixture thickens.

Remove saucepan from heat and stir in 1/2 cup peanut butter. Strain mixture through a fine-mesh sieve into a large bowl.

Let mixture cool slightly, stirring often, then stir in the heavy cream and vanilla. Cover bowl and chill in refrigerator 8 hours or overnight.

Transfer ice cream to a storage container or bowl, cover and freeze at least 4 hours until hard. Serve topped with more Rice Krispies cereal, chopped peanuts and/or chocolate sauce, if desired.