Heat oven to 350°F (175°C, gas mark 4). Grease pan and dust with flour; set aside. In large bowl, combine flour, sugar, baking powder, baking soda, and salt; set aside. In medium bowl, combine melted butter, sour cream, eggs, and vanilla; mix until well blended. Add sour cream mixture to flour mixture and blend on low speed, scraping bowl often. Beat on medium speed 1 minute. Stir in 2 tablespoons rainbow sprinkles. Carefully spoon batter into pan, dividing evenly among cups. Spread batter out to top edges of each cup, so batter is lower in the center. Bake at 350°F for 18-24 minutes, until toothpick inserted comes out clean. Cool 10 minutes in pan; invert cakes onto cooling rack and cool completely. In large bowl, combine vegetable shortening, butter and vanilla. Gradually add powdered sugar, 1 cup at a time, scraping bowl often. Add milk, beating on medium speed, until light and fluffy; color frosting as desired. Decorate with assorted sprinkles. Makes 8 cakes.