Preheat the oven to 350°F. Grease a 9'' tart pan with unsalted butter.
Place the pretzels in a food processor.* Pulse the pretzels until you have fine crumbs. You should have 1 1/2 cups of crumbs total.
Add the melted butter and brown sugar. Pulse until the mixture clumps and sticks together.**
Using your fingers, press the crumb mixture evenly into the bottom and sides of the pan. Bake for 9-10 minutes, until golden brown. Let cool while you make the caramel.
Peanut Caramel Filling
Combine the sugar and water in a heavy-bottomed saucepan. Cook over medium-low heat, stirring just until the sugar dissolves. Clip a candy thermometer to the side of the pan, ensuring that the bottom of the thermometer is hitting the sugar-water mixture but not touching the bottom of the pan.
Let the sugar-water mixture cook over medium-low heat, swirling the pan often until the mixture turns a dark amber color and the temperature reaches 350-360°F. Resist cranking the heat or stirring the mixture or it could crystallize!***
Remove the pan from the heat, and quickly stir in the vanilla extract (the mixture will bubble and splatter). Add the butter one chunk at a time until combined. Next, add the Kosher salt and heavy cream, quickly stirring to combine everything. Let the caramel cool for about 5 minutes.
Sprinkle the chopped peanuts evenly over the top of the tart crust. Immediately pour in the caramel, using a rubber spatula to spread it evenly if necessary. Pop the tart in the fridge to set, at least 30 minutes but preferably at least 1 hour.
Place the chopped chocolate in a medium heat-proof bowl.
Heat the heavy cream in a small saucepan just until it comes to a simmer. Don't let it boil! Pour the hot cream over the chocolate and let it sit for 2-3 minutes.
Stir the mixture until you have a thick, glossy ganache. Quickly spread the chocolate ganache over the tart with a rubber spatula or offset spatula.
Decorate the tart with chopped peanuts, pretzels, and/or flaky salt, as desired. Let the ganache set for 15-20 minutes. Slice and serve immediately, or pop the tart in the fridge until you're ready to serve. Let it warm up for 15-20 minutes at room temperature to allow the caramel and chocolate to soften before slicing. Enjoy!
*A note on the food processor: If you don't have a food processor, just place the pretzels into a plastic bag and use a rolling pin to break up the pretzels. Then, add all of the crust ingredients together in a bowl, mixing it up with a wooden spoon.**A note on the crust: If the crumb mixture is too dry to "clump" together, add more butter 1 Tablespoon at a time, just until it comes together. Avoid adding too much butter or it will make the crust greasy.***A note on the caramel: If you notice crystals starting to form on the side of the pan, use a wet pastry brush to brush them off.