Preheat the oven to 375°F.Pulse the graham crackers in a food processor until they reach fine crumbs. Pour in the melted butter and pulse to combine.Line a muffin tray with paper liners. Using your fingers or the back of a spoon, push an equal amount of the crumb mixture into the bottom of each cup. Bake the cups for 15 minutes, or until nicely toasted and brown. Cool completely either on the counter or by placing the whole tray in the freezer.Place the chocolate in a small saucepan and melt over medium-low heat. Once melted, slowly drizzle 1 teaspoon of the whipping cream in while stirring consistently. The chocolate will seize up into a solid mass! If it doesn't, drizzle the tiniest bit more cream in. Remove from heat.Using a stand mixer fitted with the whisk attachment, beat the remaining cream and vanilla until it holds stiff peaks. Scoop the cream into a separate bowl and put to the side.Scoop the chocolate mass into the bowl of a stand mixer fitted with the whisk attachment. Beat the chocolate on high until it becomes smooth and light, scraping down the sides and bottom of the bowl as necessary. Beat in the peanut butter to combine. Add 1/4 of the cream and a pinch of salt and beat until light and fluffy. Remove the bowl from the mixer and fold in the rest of the cream using a rubber spatula, being careful not to deflate it.Scoop an equal amount of mousse into each cup. Return to the freezer for at least 2 hours and peel off the paper liner before serving. Note: If you don't have time to freeze these, you can make cups instead of tarts. (This method yields fewer servings.) When adding crumb mixture to the paper liners, push it up the side to make a cup shape. These will be ready to serve right away after you add the mousse to each cup.