Preheat oven to 350°F. Line muffin pans with 18 cupcake liners. Sift cake flour, baking powder, and salt into a medium bowl. Set aside. Measure milk and add vanilla. Set aside.

Place butter and sugar in a large bowl. With an electric mixer, beat on medium-high until butter is pale and fluffy. Reduce speed to low. Gradually add half the flour mixture and half the milk mixture. Let it combine, then add the rest of the flour and milk. Turn mixer off.

In another large bowl, add egg whites. With an electric mixer, beat egg whites on medium-high until slightly stiff peaks form. Do not over beat. It will be very foamy but not glossy. Fold egg whites into cake batter.

Pour batter into prepared muffin pans. I like to use a large ice cream scoop to measure the batter evenly. You should have 18 cupcakes. Bake in preheated oven for 18-22 minutes or until light golden brown. Let cool.

Beat softened butter on medium speed with an electric or stand mixer. Beat for 3 minutes until smooth and creamy. Add powdered sugar, cream, and vanilla extract with the mixer running on low. Increase to high speed and beat for 3 minutes. Add green and yellow food coloring to make the icing lime green. Pour into a large pastry bag fitted with a large round pastry tip such as a 1A.

Use the bottom of a large pastry tip to cut a piece of cupcake out of the top of each cupcake. Pour green pudding into a large zip-top bag. Seal bag and cut a small corner, then pipe a little bit of pudding into the center of each cupcake. Replace the missing section of cake. Pipe buttercream on top in a swirling pattern. Add yellow M&M's® with eye drawn on using an edible black marker. Serve immediately or store in the refrigerator for up to 3 days.