In a standing mixer, cream butter and sugar. Beat in the eggs, sour cream, and vanilla. Add 1 cup flour with salt and soda, mix, and continue to mix and add 1 cup flour at a time.
Cut two large pieces of plastic wrap, divide dough in two and empty dough onto each piece, shape into disks, wrap up, and place in the fridge for at least 1 hour.
Preheat oven to 350 degrees F.
Empty dough onto a lightly floured surface and roll out to ⅓ inch thick.
Cut dough with cookie cutters and place on an ungreased cookie sheet (about 12/sheet). Bake for 8-10 minutes.
Leave cookies on pan for about 5 minutes and then remove and let cool on wire rack or towel. Frost and sprinkle when cool.
For the Frosting:
With an electric mixer, cream butter. Add half the sugar a little at a time (so you don't get a sugar cloud in your face) and blend well. Beat in vanilla and half the cream. Mix in the remaining sugar and then the remaining cream as needed to get desired consistency.
Divide into separate bowls add food coloring to reach desired colors. Spread over cookies using a butter knife or cake spatula. Pipe on letters. (It's easier to pipe on the letters right after frosting to the cookie, before the top dries over.)