These homemade mini Bundt cakes make great wedding favors when packaged in individual gift boxes.
Make the cakes: Preheat oven to 350 degrees. Butter 6 mini bundt molds. Dust with flour, and tap out excess. Whisk together flour, baking powder, 1/4 teaspoon salt, and the coconut.
Beat butter and sugar with a mixer on medium-high speed until pale and fluffy, about 3 minutes. Reduce speed to low. Add flour mixture in three additions, alternating with the coconut milk and vanilla. Fold in macadamia nuts.
Beat egg whites and a pinch of salt with the mixer's whisk attachment until stiff peaks form. Fold into batter. Divide among prepared molds, filling each about three-quarters full. Bake until a toothpick comes out clean, about 30 minutes. Let cool slightly in molds on wire racks. Turn out cakes onto rack.
Make the glaze: Stir together rum and confectioners' sugar until smooth. Brush over warm cakes.