Before you start: Put your butter in the fridge or freezer to ensure it's as cold as possible. Do this with your ice water as well.
Add flour and salt to a bowl – mix. Add cubes of very cold butter and using your hands toss until all the butter cubes are evenly coated in flour. Squish each cube of butter between two fingers and toss in flour again. Gently rub the butter into the flour until the butter is about the size of walnut halves or leave as is for an extra flaky pie crust.
Create a well in the middle of the flour and pour in about half the cold water and toss until all the water is absorbed into the flour. One tbsp at a time, add more cold water to the bowl and mix until the dough comes together – you want to handle it as little as possible. You may need to add more or less water than stated as different flours hydrate at different percentages.
Divide the dough into quarters, form into a disk and wrap in plastic. Chill for at least 30 minutes. If making this recipe across a couple days this is a good place to stop.
Flour the work surface and roll chilled dough out to 1/8 "- 1/4" thickness depending on how thin you want the shell. Using a 4.5" round cookie cutter, cut each rolled out dough disc into 3-4 circles.
Grease a muffin tin and gently press the circles into each muffin cup and crinkle the edges, it should be sticking out of the muffin cup. Repeat for remaining dough and chill the muffin tin while you prepare the filling.
Before you start: Make the flax egg by combining 2 tbsp ground flaxseed and 3 tbsp warm water and set aside to thicken.
Preheat oven to 400F.
Add vegan butter and brown sugar to a small saucepan over medium heat, stirring until the mixture is bubbling. Remove from the heat and set aside.
Whisk flax eggs, maple syrup, vanilla, and salt in a medium-sized bowl. Pour in the butter mixture slowly while whisking constantly to incorporate.
Make Pecan Butter Tarts:
Sprinkle pecan pieces into the bottom of each tart and pour in the filling until 3/4 full.
Bake at 400°F for 10 minutes, then reduce the heat to 350°F and bake for an additional 10-15 minutes until the filling in bubbling and he crust is golden.
Cool the tarts for 5 minutes, then give them a gentle twist in the pan to ensure they aren't stuck anywhere. Allow to cool completely in the pan and enjoy.