In a food processor with a whip attachment or your stand mixer, whip the buttercream for 30-40 seconds.
1 ¼ Cup - 225 grams Leftover or Split Buttercream at room temperature.
The buttercream would be smooth enough.
Now add the flour, ground hazelnut, and vanilla extract.
1 ½ Cup - 290 grams Whole wheat flour, 1 teaspoon Vanilla Extract
Whip it again or add all the ingredients in a bowl and combine.
Form a dough. If you are unable to form a dough, add a tablespoon of milk or cream at a time.
4 tablespoon Milk/Fresh Cream
Combine until you get soft and smooth cookie dough.