Spray a tart pan(s) with a removable bottom(s) or pie plate with some baking spray and set aside.
Crush the pretzels in either a food processor with the pulse feature until you get coarse crumbs. You can also put the pretzels in a sealed ziploc and use a rolling pin to crush the pretzels until you get the desired texture. (The second option is a good stress reliever!)
In a medium bowl, stir together the pretzel crumbs, butter, sugar, and flour until well combined.
Pour the crust mixture into the prepared tart pan and use the bottom of a measuring cup to press the crumbs evenly into the pan.
Bake for 10 minutes in a preheated oven until golden. Remove and set aside to cool on a rack.
For the mousse
In a clean mixing bowl, whip the heavy cream with a hand or stand mixer on medium high until you get stiff peaks (about three minutes*). Tip: chill the beaters and bowl to make the cream whip faster.
Gently add the peanut butter to the bowl and using a rubber spatula, fold the peanut butter into the whipped cream. It will eventually all incorporate (just don't stir like crazy but gently fold). For more detailed information, check out the Peanut Butter Mousse recipe.
Chill the mousse if you aren't ready to assemble the bars.
If you aren't using the ganache until the next day, refrigerate overnight and bring to room temperature before spreading on the tart. You can also quickly microwave in 10 second increments stirring between to soften. BE CAREFUL not to burn the ganache or you will be starting over.
For the ganache
Heat the heavy cream in a small saucepan over medium heat until small bubbles appear on the outside. Don't boil the cream! Remove the pan from the heat and add the chocolate stirring with whisk until the chocolate is completely melted and the ganache is thicker.
Assemble the bars
Spread the mousse over the pretzel crust to the edges.
Freeze for 4 hours or overnight.
See tips below for slicing/serving.
SERVING OPTION #1: Remove the pan from the freezer. Slice into bars, remove to plates and pour the ganache over. Allow guests to decorate their own bars with assorted candy and nut toppings.
SERVING OPTION #2: Pour ganache over the whole tart as shown, decorate and refrigerate/freeze until the ganache hardens. Remove and let sit for about 10 minutes before slicing into bars, triangles or any shape you desire.
*Stiff peaks are when the beaters are lifted out of the bowl and the resulting "peaks" stand straight up instead of curling or flopping over. The cream will be very thick.Serving/slicing tips
Remove the pan from the freezer and let the pan sit for about 10 minutes.
Run a butter knife under warm water. Dry it and then run it around the edges of the tart pan to loosen it from the edges. Push up bottom center of the tart pan and lift the tart up through the pan and use your other hand to catch it.
Use a sharp knife and slice the tart into your desired size. I love cutting it crosswise into triangles for a fun presentation.
Heat the ganache and pour into a bowl. Offer guests a topping bar to customize their chocolate peanut butter pretzel bars.