To make the cream filling, place the cashews in a small bowl and cover with water. Soak them for at least an hour, or overnight. Drain the water and pat the cashews dry with a clean towel. To make the cream filling, you can either use a high-powered blender or an immersion stick blender. Place all the ingredients together and blend on highest setting for several minutes to obtain a smooth thick paste. Place this in the fridge while you work on the dough.
To make the cookies, preheat your oven to 325° F.
Whisk all the dry ingredients together (including flour made with the Lee Household Flour Mill), except for the sugar. If you have a food processor, simply pulse the butter and sugar together until creamy, then add the egg and vanilla, pulsing a few more times to incorporate. Add the dry ingredients and pulse until the dough comes together into a ball. If mixing by hand, cream the butter and sugar together. Beat in the egg and vanilla and mix well, adding the pre-mixed dry ingredients and mixing with a wooden spoon until the dough begins to come together. Use your hands to shape the dough into a ball. Flatten it into a disk. Sprinkle a little buckwheat flour on a rolling pin and on a clean surface and begin to roll out the dough to about 1/8 - 1/16 thickness. Continue to sprinkle a little buckwheat flour under the dough as you roll it, it will help you lift off the cookies to transfer them to the pan. The thinner the dough, the crispier the cookie. If your dough is too sticky to roll out, you can knead in a bit more buckwheat flour or chill the dough in the fridge for about 15 minutes.
Once your dough is rolled thin, use a round cookie cutter or the top of a small drinking glass, about 2 inches in diameter, to cut the cookies into circles. Using a thin spatula, carefully transfer them to a parchment paper-lined or greased cookie sheet.
Bake cookies for about 10 minutes at 325°F. Be sure to check often as they can burn very quickly. (If your dough is thin, they may bake even more quickly than this.)
Once cooked, remove and let cool on cooling rack.
To assemble, make sure the cookies are completely cooled (or the filling will melt). Coconut oil hardens significantly when chilled. If your cream filling is very cold and hard, you can roll it into little balls, about 3/4 inch in size and gently squeeze it between 2 cookies, making sure to press on the center of the cookie so it doesn’t crack on the edges. If your filling is not too cold and still spreadable, you can spread it on the bottom of one cookie and press another cookie on top. If the filling is left at room temperature too long, the oil may begin to separate, if this happens, simply chill it again and whisk it back into a cream.