Preheat oven to 375º. Sift together sugar, flour, and salt in a bowl. Combine lemon juice, vanilla extract, rum, and orange extract.
Beat egg whites and cream of tartar at high speed with a heavy-duty electric stand mixer until stiff peaks form; gently transfer egg white mixture to a large bowl.
Gradually fold in sugar mixture with a large spatula, 1/3 cup at a time, folding just until blended after each addition. Fold in lemon juice mixture.
Arrange 12 (2 1/2- x 2-inch) muffin-size paper baking molds* on an aluminum foil-lined baking sheet; spoon batter into baking molds, filling almost completely full.
Bake at 375º for 17 to 19 minutes or until a long wooden pick inserted in centers comes out clean. Transfer to a wire rack, and cool completely (about 1 hour).
Make a small hole in top of each cupcake using the handle of a wooden spoon. Spoon Lemon Curd Filling into a zip-top plastic freezer bag. Snip 1 corner of bag to make a tiny hole. Pipe a generous amount of filling into each cupcake.
Spread Cream Cheese Frosting on tops of cupcakes. Garnish, if desired.
*15 jumbo aluminum foil baking cups may be substituted. Place baking cups directly on an aluminum foil-lined baking sheet; fill cups, and proceed as directed.