Cut the butter into small cubes and place in a large bowl. I cut them into tablespoon-sized squares and cut each square into 9 little squares.
Measure out 2 cups of flour. Sift the flour over the butter.
Use your fingers to massage the butter into the flour, leaving chunks of butter unincorporated.
Add the tablespoon of vinegar and toss. Add the ice water a tablespoon at a time until the dough is able to hold together when pressed. It should be very cool to the touch, moist enough to hold together, but not wet enough that excess moisture is left behind in the mixing bowl or on your hands.
Form into a ball and wrap in plastic wrap. Store in the refrigerator for 30 minutes.
Dust the counter with flour. Roll the dough out until it is about 1/4 of an inch thick.
Use the edges of your mini tart pans to cut out 6 circles in the dough. Use your rolling pin to roll the circles out till they are larger than the tins. Place all the pie tarts onto a baking sheet.
Place the circles of dough in the tart pans and press the dough into the sides of the pan. Put the baking sheet in the fridge and let it rest for 30 minutes. This will help prevent the dough from shrinking during the blind bake.
While the dough is chilling, preheat the oven to 450 degrees Fahrenheit.
Bake the mini tarts for 10 minutes, occasionally checking to make sure there are no air bubbles. If an air bubble starts to expand, prick it with a fork. You can use pie weights if you want, but I don’t find them necessary.
Take tarts out and let them cool completely while you prepare the marzipan and chia seed jam.
Combine the rhubarb, orange zest, and honey in a saucepan and cook on medium heat 1 to 2 minutes until the rhubarb has softened a little and is mostly submerged in juices.
Add the chia seeds and reduce heat to medium-low and cook for 2 to 3 minutes.
Stir in the orange juice, Amaretto, and salt and cook for 2 to 3 more minutes. Turn off the heat and add a couple dashes of Orange Bitters.
Pour the jam into a mason jar and let it come to room temperature before putting on the lid and storing in the fridge.
Combine all the ingredients in a food processor and mix until everything comes together smoothly.
Preheat oven to 350 degrees Fahrenheit.
Place 1/4 cup marzipan in each tart shell*
Place 2 tablespoons of the rhubarb chia seed jam on top of the marzipan.
Bake for 40 to 45 minutes, until the crust is a light golden color and the rhubarb jam has darkened to a deep red-brown color.
*Spreading the marzipan into the tart shells is difficult so I recommend piping the marzipan into the shells.