Mix the flour, baking powder, baking soda, and salt in a medium-sized bowl and set aside.
In a large bowl, with an electric mixer on medium speed, beat the eggs and sugar for about 2 minutes, or until light and creamy.
Add the butter and vanilla and beat on low speed for about 1 minute, or until well blended.
Beat in the dry ingredients on low speed until blended.
Add the sour cream and beat until smooth and well blended.
Fill 6 cupcake liners about halfway full.
Melt the chocolate (either using a double boiler or in the microwave).
Mix the chocolate into the remaining batter.
Fill the remaining 6 cupcake liners with the chocolate batter.
Bake at 350 F for about 23 minutes, or until a toothpick inserted into the center of a cupcake comes out clean and the tops are firm.
Sour Cream Frosting While you can top these cupcakes with any frosting, you might want to try a sour cream frosting. The sour cream adds a nice tang to a sweet buttercream.