Preheat the oven to 350 degrees F. Lightly grease a 9-inch tart pan.

To make the chocolate stout mousse, add chocolate and stout to a large heat-proof bowl. Place the bowl over a pot of simmering water, making sure that the bowl doesn't come in contact with the water. Allow the chocolate to melt, stirring often, until smooth. Remove from the heat and allow the chocolate to cool while you prepare the crust.

To make the crust, add pretzels, peanuts, and brown sugar to the bowl of a food processor. Pulse until the mixture resembles a fine crumb. Slowly drizzle in the melted butter while simultaneously pulsing. Continue to pulse until the mixture resembles wet sand. Transfer the mixture to your prepared tart pan. Press the mixture firmly into the pan and slightly up the sides. I find that this is easiest with a measuring cup or small glass. Bake the crust for 10-12 minutes, or until golden brown. Cool completely before adding the mousse.

Add the heavy cream to a stand mixer fitted with the whisk attachment (or in a large bowl using a handheld mixer). Beat until soft peaks form. Add the espresso powder, powdered sugar, and vanilla extract. Continue to beat until stiff peaks form. Gently fold in the cooled chocolate. Transfer the mousse to the tart pan. Garnish with shaved chocolate. Cover and refrigerate for at least 2 hours before serving. The tart will stay fresh in the refrigerator for up to 5 days.