In mixing bowl, cream together butter and brown sugar until smooth and creamy. Mix in egg, then mix in sour cream and vanilla extract.
In medium bowl, whisk together flour, malted milk powder, salt and baking soda. Gradually add and beat in with butter and sugar mixture. Cover dough with plastic wrap and chill 20-30 minutes in refrigerator.
On well-floured surface, roll half of the dough out to 1/4-inch thickness. Cut with a round cookie cutter and place cookies an inch apart on a non-stick cookie sheet or one lined with parchment paper or a Silpat. Repeat with other half of the dough.
Bake Whoppers cookies 9-11 minutes or until edges are golden. Remove to wire racks to cool completely.
To Make Frosting
In mixing bowl, beat powdered sugar, butter, malted milk powder, vanilla extract and milk until smooth and creamy. Add milk if needed to reach desired consistency. Spread frosting over cooled cookies and sprinkle chopped Whoppers malted milk balls on top.
One 5-oz box of Whoppers is all you need for this recipe.
Be careful when chopping the malted milk balls; they're slippery little suckers.
Chill dough 20-30 minutes before rolling out. The dough is sticky so make sure to flour your workspace well. Roll dough out to 1/4-inch thickness.