Preheat oven to 350°F. Grab 4(4-inch diameter) baking paper cups, or 12 standard muffin paper liners and set aside. You can also use a 7- to 9-inch springform cake pan greased and lined with a circle of parchment paper.
Add the sunflower seeds into a skillet and toast the seeds over medium heat for 5 to 8 minutes, until lightly golden and fragrant. Remove from heat.
For the crust: Add the toasted sunflower seeds, oats, and salt into a food processor and process until finely chopped, resembling a coarse flour. Now, add the melted oil, maple syrup, and sunflower seed butter, and process until the mixture comes together into a heavy dough. The dough should stick together very easily when pressed between your fingers. If it is still dry, add the optional water and process again until it comes together.
Divide the dough equally between the liners, crumbling it all over each base. (Note: I reserved about 1/2 cup of the dough for sprinkling on the pies when serving.) Lightly wet your fingers. Starting at the centre, press the dough into the base to form a crust. If the dough starts to stick to your hands at any time, lightly wet your hands and shake off the excess water.
Once the crust is all pressed in, poke the base with a fork about 2 to 3 times to allow the air to escape while baking. I place the 4 liners onto a baking sheet so it's easy to remove them from the oven.
Bake the crusts for around 10 to 12 minutes until lightly golden around the edges. Place baking sheet on a cooling rack and cool the crusts for at least 15 to 20 minutes, or longer if you have the time.
For the filling: Open the chilled can of coconut milk. Scoop off 1/2 cup of the white coconut cream only (not the water) and place it into a blender. (You can save the remaining contents of the can for a smoothie or other use.)
Add the pumpkin purée, maple syrup, melted oil, sunflower seed butter, vanilla, pumpkin pie spice and salt. Blend, starting at a low speed and increasing the speed, until smooth.
Divide the filling equally among the cooled crusts. Carefully, transfer the baking sheet (or muffin tin, if making 12 mini tarts) into the freezer on a flat surface. Chill until firm, about 3 to 5 hours. No need to cover it.
After the tarts are solid, carefully remove the paper liners. Here you can transfer the pies to the fridge for a mousse-like texture, but I prefer the texture when frozen after sitting on the counter for 5 to 10 minutes before serving.
Serve with coconut whipped cream on top, if desired. Return to the fridge or freezer for best results.