Blend the cottage cheese for the filling until completely smooth. Add the cream cheese, protein powder, powdered sugar, vanilla and salt, then blend until thick and fluffy.Transfer the filling to a bowl, cover and chill for at least 1 hour so it firms up to a pipeable consistency.
Preheat the oven to 350°F and line 2 baking sheets with parchment paper. Blend the cottage cheese for the cookies until smooth.
In a large bowl, whisk the blended cottage cheese, melted butter, brown sugar, molasses, egg and vanilla until smooth. Add the flour, protein powder, oats, cinnamon, baking soda and salt, and stir until fully combined.Let the dough rest for 10 minutes so the oats soak up moisture and the dough thickens.
Scoop 24 rounded tablespoons of dough onto the baking sheets, spacing them 2 inches apart, and flatten each one slightly. Bake for 9 to 11 minutes, until the tops are set but still soft.
Cool on the baking sheets for 5 minutes, then move to a wire rack and cool completely.
Pipe or spread a thick layer of the cream filling onto the flat side of half the cookies and top with the remaining cookies. Chill for 30 minutes to set, then store in the fridge.