Line a 9x13 baking pan with foil and spray with non-stick cooking spray.
Combine butter and sugar in a bow and mix with electric mixer until light and fluffy, about 2-3 minutes.
Add egg, sour cream and vanilla to mixture and beat until combined.
Add flour, baking powder and salt to mixture and stir until just combined. Do not over mix.
Scoop dough into baking pan and use fingers to distribute dough over bottom of pan.
Bake for approximately 15 minutes or until the center appears set and sides are starting to lightly brown. Do not overbake. Remove from oven and allow to cool completely prior to frosting.
To make frosting, combine softened butter and powdered sugar in a bowl and mix with electric mixer until well combined. Add milk and vanilla and mix until well combined. If a thinner frosting is desired, add more milk, 1 teaspoon at a time until desired consistency is reached. If single colored frosting is desired, add food coloring then spread frosting over bars and skip to #11.
If marbled frosting effect is desired, divide frosting evenly into four bowls. Use food coloring to color frosting in each bowl to desired shade. Transfer frosting from each bowl into it's own sandwich style bag. Cut a small corner off of each bag to allow frosting to be piped onto bars from bags.
10. Using bags to pipe, place small dollops of different colored frosting all over bars. Using an offset spatula, very gently spread dollops together. Do not over spread or you will lose the marbled color effect in the frosting.
11. Gently press Brach's Conversation Hearts into frosting, then cut and serve.
Notes: Make sure not to overbake as these bars keep baking even after removed from the oven.