In a medium bowl, combine 5 cups of flour, baking soda, baking powder and a pinch of salt. Mix and set aside.
In a stand mixer bowl, cream the butter and granulated sugar at medium speed until light and fluffy for a few minutes. In the same bowl add one egg at a time. Continue to mix until everything is combined well. Incorporate the vanilla extract and sour cream and beat at low speed until combined.
Now slowly add the dry ingredients and sprinkles and beat at low speed until just combined, about half a minute. Add the remaining 1 cup of flour to the dough, starting with 1/3 cup. Divide dough into two sections. Flatten into rectangles about 1 1/2 inches thick using the straight edge of the counter, then wrap with plastic wrap. Place in refrigerator to chill over night.
Line 2 large baking sheets with parchment paper or spray them with nonstick cooking spray, set aside.
Preheat the oven to 425 degrees F.
Flour a work area and rolling pin. With a rolling pin, roll the dough out to 1/4-inch thickness. Use a glass rim or a 2 1/2-inch round cookie cutter to cut out circles and transfer to a baking sheet. Bake for 7-8 minutes, until pale golden. Immediately transfer cookies to a wire rack to cool. Wait at least 30 minutes before frosting.
In a stand mixer cream together the butter and vanilla. If the butter is not soft, simply place in the microwave for about 15 seconds. Slowly beat in the powdered sugar. Once desired consistency is achieved, mix in heavy cream, 1 tablespoon at a time until the desired spreading consistency is achieved. It should be soft and smooth just like Lofthouse sugar cookie frosting.
Once cookies have cooled completely, about 30 minutes, frost and add sprinkles. Allow frosting to set, then store in an air-tight container. Let cookies sit overnight before serving to allow the flavors to develop. The cookies can store in an air-tight container for about 1 week. Also, these cookies freeze well for about 3 weeks.
Makes 4 dozen cookies