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 1 round of dough onto a lightly floured surface and roll out to 1/3 inch thick. Cut out dough with cutters getting as close together as possible to minimize scraps. Place on an ungreased pan 2 inches apart and set scraps aside. Roll out and cut second round of dough. Add the scraps to the other scraps and knead it together. Roll out and repeat until all the dough is used.
Bake one pan at a time in the middle of the oven for 10 minutes. Cookies will look slightly puffed and white. They should not be golden at all.
Leave cookies on pan for about 5 minutes and then remove and let cool on wire rack or towel. When completely cooled, frost and immediately add sprinkles and candy.
For the Frosting:
With an electric mixer, beat the butter. Add half the sugar and blend well (start off slow!). Beat in vanilla and cream. Mix in the remaining sugar as needed to get desired consistency.