Combine milk, half-and-half, and cocoa powder in a pot over medium heat. Cook until heated through, reaching about 130 degrees F (54 C) but not boiling. Stir occasionally to break up lumps and prevent a skin from forming on the surface.

Meanwhile, combine egg substitute and sugar in a separate bowl. Whisk until sugar is incorporated. Pour in a little of the hot milk mixture to temper the eggs; mix well and add back to the mixture on the stove. Reduce heat to medium-low. Stir until mixture reaches 185 degrees F (85 C) and thickens enough to coat the back of a spoon, but do not let boil.

Turn off the heat. Add peanut butter, vanilla extract, and salt. Stir until melted and combined. Let ice cream base cool, stirring occasionally, until steam stops rising, at least 20 minutes. Place pot over an ice water bath if desired. Cover and refrigerate, 8 hours to overnight.

Pour base into an ice cream maker and freeze according to manufacturer's instructions until texture resembles soft-serve, about 20 minutes. Transfer to an airtight container and freeze until firm, about 4 hours.