Line a muffin tin with cupcake papers. Heat the oven to 350 degrees F. Sift the already sifted cake flour with the baking soda, baking powder and salt. Cream the butter in a mixer fitted with a whisk attachment until soft, then add the sugar and mix. Add the yolk, the egg and the vanilla and whip at medium-high speed until light and fluffy. With the mixer running at low speed, add a third of the flour mixture and mix. Then add half of the sour cream and mix. Add another third of the flour and mix. Add the remaining sour cream and mix, then the remaining flour. Give it one last mix to make sure everything is blended in. Pour the batter into the muffin cups, filling them 3/4 full. Bake until firm to the touch in the center, 20 to 25 minutes. Set the pan on a wire rack and let cool.
To make the butterflies, spread a thick layer of rainbow sprinkles on a plate. Cut off the top rounded "cap" (like the cap of a mushroom or a muffin) of each cupcake. Spread the flat top of each "beheaded" cupcake with frosting. Cut a "V" shape wedges out of the center of each cap, creating 2 semicircles that will serve as the butterfly wings. Spread the flat cut side of the 2 semicircles with frosting, then dip in rainbow sprinkles to coat. Place the 2 wings on top of each cupcake, sprinkles side up, so that the curved sides meet in the middle.
Lay 3 strands of red licorice down the center of each one to make the body. Use two pieces of black licorice to make the antennae. Pipe 2 black dots for the eyes, placing one white sprinkle in the center of each eye.
Over medium heat, cook the flour and milk until thickened. Let cool. In the bowl of an electric mixer, cream together the butter, sugar and vanilla until light and fluffy. Beat in the flour mixture.
Recipe courtesy of "Gale Gand's Just a Bite: 125 Luscious Little Desserts" by Gale Gand and Julia Moskin: Clarkson N. Potter Publishers, 2001