For the cookies, preheat the oven to 325F and line a large baking sheet with parchment paper or a silicone liner.
In a medium bowl, whisk together almond flour, Muesli, cinnamon, baking powder and salt.
In a large bowl, cream butter with granulated erythritol and molasses. Beat in egg until well combined. Beat in flour mixture until dough is well mixed.
Shape dough into 1-inch balls and place 2 inches apart on prepared baking sheet. Press each ball with the heel of your hand to ¼ inch thickness.
Bake 12 to 14 minutes, until just beginning to brown and barely firm to the touch.
Remove from oven and let cool on pan 5 minutes, then transfer to a wire rack to cool completely.
For the filling, beat butter until smooth, then beat in powdered erythritol. Beat in cream and vanilla until a spreadable consistency is achieved.
Take one cookie and spread the bottom with filling, then top with another cookie, bottom-side down to create a sandwich. Repeat with remaining cookies and filling.