Place 3 ounces cream cheese in the bowl of a stand mixer (or large bowl if using an electric hand mixer). Let sit until softened, about 1 hour.

Coarsely chop 2 ounces semisweet chocolate. Pour ½ cup heavy cream into a medium microwave-safe bowl and microwave until very hot and bubbling gently, 40 seconds to 1 minute. Add the chocolate and make sure the chocolate is covered in the cream. Let sit for 1 minute. Stir until the chocolate is fully melted and combined.

Add ¾ cup powdered sugar, ⅓ cup packed light brown sugar, ½ teaspoon vanilla extract, and ¼ teaspoon kosher salt to the cream cheese. Beat with the whisk attachment on medium speed, scraping down the sides of the bowl halfway through, until combined and the sugar is dissolved, about 1 minute total.

Reduce the mixer speed to medium-low. Slowly pour in 1 ¼ cups cold heavy cream. Increase the speed to medium-high and beat, scraping down the bowl as needed, until stiff peaks form, about 1 minute.

Add ⅔ cup creamy peanut butter and beat on medium-low speed until fully incorporated and fluffy, about 30 seconds more. The mixture will look broken at first and then smooth out.

Divide the mousse between 4 (6 to 8-ounce) coupes, jars, or ramekins (about ¾ cup each). Gently smooth the top of each with the back of a spoon. Give the ganache a good stir. Spoon about 2 tablespoons of the ganache over each portion of mousse and gently swirl the glass to coax into an even layer so as not to disturb the mousse. (A wider mouth, shallow glass such as a coupe will need the full 2 tablespoons of ganache to cover the top. If using something like a narrow jar, you might not need all the ganache for the top. Instead, consider layering the ganache in the jar with the mousse.)

Garnish with chopped roasted peanuts. Refrigerate until well chilled and set, at least 4 hours and up to overnight. Let sit at room temperature for 5 to 10 minutes before serving.