FOR COOKIES: 1½ cups (150 g/5.3 oz) almond flour  1/3 cup (50 g/1.8 oz) erythritol or Swerve  ½ teaspoon baking soda blended with 1 teaspoon cream of tartar, or 1 teaspoon gluten-free baking powder  Pinch salt 1 teaspoon sugar-free vanilla extract or  ½ teaspoon vanilla powder 1 teaspoon freshly grated lemon zest 2 massive egg yolks 2 tablespoons (30 g/1.1 oz) butter or  ghee, at room temperature  FOR COOKIE BUTTER: Crushed cookies (recipe follows) ½ cup (112 g/4 oz) unsalted butter  or ghee or coconut oil, at room temperature  ½ cup (90 g/3.2 oz) darkish chocolate chips, 85 p.c cacao