Whisk together the flour, baking powder, baking soda, and salt. Set aside.

Using an electric mixer on medium speed, beat the butter, brown sugar, and sugar until light and fluffy. Add the eggs, one at a time, mixing well after each addition. Mix in the vanilla.

Reduce mixer speed to low. Gradually add the flour mixture, mixing just until combined. Stir in the macadamias.

Scoop the dough into 1-tablespoon portions. Roll each portion between the palms of your hands to form a ball. Place the dough balls on the prepared pans. Flatten the top of each ball slightly.

Bake 12 to 14 minutes, or until the edges are lightly browned and the tops appear set.

Cool on the pans for 10 minutes. Then transfer the cookies to a wire rack to cool completely.

Using an electric mixer on medium speed, beat the butter and confectioners' sugar until light and fluffy. Add the cooled white chocolate and vanilla, and mix until thoroughly combined and smooth.

Spread frosting on the bottoms of half of the cooled cookies. Top with the other cookies so that the bottoms of the cookies are facing each other.