In a high-speed blender, combine the macadamia nuts, cashews, 2 tablespoons of almond milk, maple syrup, coconut oil, vanilla, lemon juice, and salt. Blend until smooth, adding up to 2 tablespoons more almond milk, if necessary, to blend. Use the blender baton to help blend. Chill the frosting while you bake the cookies so that it firms up. (Frosting can be made ahead and stored in the fridge for up to 6 days.)
Preheat the oven to 350°F and line a large baking sheet with parchment paper. Use a food processor to combine the all-purpose flour, almond flour, sugar, baking powder, and salt. Add the coconut oil and pulse until combined. Add the almond milk and lemon zest, and pulse until combined.
Scoop the dough out of the food processor and knead gently to form a ball. Divide the dough into 12 equal pieces and roll each into a smooth ball. Place the balls onto the baking sheet and gently press each down to form small disks.
Bake for 12 to 15 minutes or until very slightly browned. Remove from the oven and cool on the pan for 10 minutes before transferring to a wire rack to finish cooling. Let the cookies cool completely before frosting.