The Chocolate Sauce

1. In a medium sauce pan, combine the milk, dark brown sugar, maple syrup and cocoa powder. Bring to a boil on meadium heat, whisking frequently. Remove from heat and whisk in bittersweet chocolate, vanilla and salt. 
2. Taste, adding more maple syrup if you want it bit sweeter and more chocolate if you want it a bit thicker. The chocolate sauce will thicken as it cools.
3. Separate 3/4 cup and store the rest in the refrigerator. It will last up to 3 weeks. 

Chocolate sauce and pretzel layer

1. Rough chop the pretzles.
2. Rouhg cop the walnuts.
3. Combine with the chocolate sauce.
4. Set aside.

The Crust

1. On high speed in a food process, blend together the crust ingredients until it pulls away from the sides. The nuts should be finely processed. 
2. Take the tart pan disk and measure out a round piece of parchment paper and line the pan.
3. Pour the crust into a tart pan and spread out covering the entire inside. Approximately a 1/2 thickness all around.
4. Next, add a layer of the chocolate pretzel mixture. Spread it out over the bottom but not the sides.
5. Chill the crust in the refrigerator for 30 minutes

The Filling

1. In a food processor, puree the filling ingredients until creamy smooth. This will take several minutes.
2. Taste and add more zest if you want it a stronger lemon flavor, add more maple syrup if you want it a bit sweeter.
3. Remove the crust fromt he refrigerator and spread the filling over the chocolate pretzel layer and smooth out with a rubber spatula. Place in the refirgerator and chill for 2-3 hours. The filling will thicken as it chills.

The Crumble

1. In a small bowl, combine the crumble ingredients and spead out on a cookie sheet. Bake for 10-15 minutes in the oven set at 350 degrees F. Checking often. When you check the crumble, move it around with the rubber spatula.
2. Remove from the oven when the crumble turns a very light brown. Let cool.

Serving

1. Sprinkle the crumble over the top and serve!