Prep: Preheat the oven to 350°F (180°C). Spray a round 9-inch tart pan or cake pan with nonstick spray. I like to use a tart pan here because the removable bottom makes it easier to cut and serve neat slices, but you can, of course, use a cake pan if that’s what you have!
Make the Crust: Pulse the peanuts in a food processor just until they’re finely chopped. Don’t let it run too long or you’ll make peanut butter! Melt the butter in a medium mixing bowl. Stir in the crushed peanuts, flour, salt, sugar and baking powder. Scrape the bowl really well to get all the crust bits out, and press the crust mixture evenly into and up the sides of the tart pan. Bake for 9 minutes. Set aside to cool completely while you make the filling.
Make the Filling: Give the medium mixing bowl a quick wipe with a kitchen towel if there are still crust pieces in it. Measure the cold heavy cream into the bowl using a 1 or 2-cup glass measuring cup. Beat the heavy cream with an electric mixer on high speed just until softly whipped—we’re not going for stiff peaks here. Pour the softly whipped cream back into the measuring cup you used to measure it. Pop this in the fridge while you finish making the filling. Add the cream cheese to the same bowl, and beat on high speed until creamed. Add the peanut butter, sugar, vanilla and salt, and mix until well combined. Add about one-third (just eyeball it) of the whipped cream into the bowl with the filling, and mix until just combined. Add the rest of the whipped cream, and gently fold it in with a silicone spatula until it’s completely combined. If the crust is still cooling, cover the bowl with a kitchen towel and pop in the fridge until the crust is cool. Spread the peanut butter filling into the cooled crust and smooth it out. Pop in the fridge while you make the ganache.
Make the Chocolate Ganache: Add the chopped chocolate to a small bowl. Add the heavy cream to a small pot and bring to a simmer. Pour over the chocolate, and let sit for 5 minutes. Then, stir until everything is combined and smooth. If a few lumps of chocolate remain, microwave in 15 second intervals, stirring between each, until smooth. Let this cool for a few minutes more so it's a bit thicker and easier to spread, and then spoon it over the tart.
Serve and Store: Let the tart chill in the fridge for at least 6 hours or overnight before serving. Serve with a sprinkle of peanuts and flaky sea salt. Store leftovers in an airtight container in the fridge for up to 5 or 6 days.