Step 1

Preheat oven to 350°F. Line a standard cupcake pan with 10 paper liners.

Step 2

In a large bowl or stand mixer, cream together butter and sugars until light and fluffy. Mix in egg and vanilla, followed by salt, baking soda and cocoa powder. Scrape bowl as needed. Mix in sour cream, combining well before alternating additions of milk and flour. Mix until just combined and no streaks remain.

Step 3

Divide batter equally between prepared liners using a large cookie scoop. Liners should be roughly 2/3 full.

Step 4

Bake at 350°F for 18-20 minutes or until a toothpick inserted comes out clean or with a few moist crumbs. Cool in pan for several minutes or until easy to handle. Transfer to a wire rack to cool completely.

Step 5

When cupcakes have cooled completely, prepare frosting. In a large bowl or stand mixer, beat butter until light and fluffy. Add vanilla, followed by one cup of powdered sugar at a time. Scrape bowl as needed. Slowly add 1 tablespoon of heavy cream at a time until fully incorporated and desired consistency is reached.

Step 6

Divide frosting equally between six clean bowls. Tint with desired gel food colors. Transfer each color to their own small piping bag with the tip snipped off or into a quart size plastic bag with a corner cut off. Lay a single sheet of plastic wrap on flat work surface. Piping one color at a time, pipe a 1-inch thick stripe of frosting. Repeat process immediately next to the first color with next color. Repeat with remaining colors until a rainbow has been piped. Carefully pull up on plastic wrap to roll frosting over onto each other, creating a large log of frosting. Overlap plastic wrap and twist ends to seal. *This may be done in two batches of stripes/frosting logs.

Step 7

Prepare a large piping bag with desired piping tip. Cut off one twisted end of plastic wrap from prepared frosting log. Insert wrapped frosting into prepared piping bag. Frost cupcakes as desired.

Step 8

Enjoy immediately. If making ahead, cupcakes can be stored at room temperature or chilled in the refrigerator for up to several days. Chilling will cause the cupcakes and frosting to firm up. For a softer consistency, bring to room temperature prior to eating.