Heat oven to 350°F.

Spray 36 mini muffin cups with cooking spray.

In medium bowl, beat ¾ cup butter and the granulated sugar with electric mixer on medium speed 2 minutes or until smooth and creamy.

Beat in egg and ¼ teaspoon rum extract until well blended.

On low speed, beat in flour, baking soda, cream of tartar and salt about 1 minute or until dough forms.

Shape dough into 36 (1¼-inch) balls.

Place 1 ball in each muffin cup.

Bake 10 to 12 minutes or until set and edges are light golden brown.

Using end of wooden spoon handle, press 1-inch-deep indentation in center of each cookie to form a cup.

Run knife around edges of cookie cups to loosen.

Cool completely, about 15 minutes.

Remove from pans to cooling racks.

In large bowl, beat powdered sugar and ⅓ cup butter with electric mixer on low speed until well blended.

Add ⅛ teaspoon rum extract and 1 tablespoon milk; beat until smooth and spreadable, adding additional milk, 1 teaspoon at a time, if needed for desired consistency.

Stir in pineapple.

Spoon about 2 teaspoons filling into each cookie cup.

Sprinkle with coconut.

Store on plate in refrigerator, loosely covered with plastic wrap.