In a medium bowl combine 5 cups of the flour, baking soda, and baking powder; set aside.
Next, in a large bowl cream the butter and sugar in a stand mixer on medium speed using the flat beater attachment. {You'll want to mix until light and fluffy, about 3 minutes.}
Add the eggs one at a time until each is incorporated, then add vanilla and sour cream and beat at low speed until combined.
Gradually add the dry ingredients and beat at low speed, scraping the sides of the bowl as needed. Slowly add in the remaining 1 cup flour, 1/4 cup at a time, until dough is right consistency for rolling {may not need entire cup.}
Divide the dough into two sections, flatten into rectangles about 1 1/2 inches thick, then wrap with plastic wrap. Chill in the refrigerator overnight. {Cold dough is essential, but in a pinch an hour or two in the freezer will do}
When ready to prepare, preheat the oven to 425 degrees and line 2 large baking sheets with parchment paper.
When it's time to roll out the dough, I grab about ⅓ of each half at a time from the fridge. To roll out the dough further, I like to place it between two sheets of parchment paper {and lightly flour each side of the dough.} Roll out to a 1/4" thickness.
Use a cookie cutter that is approx. 2" {or adjust bake time accordingly} to cut shapes - it helps to dip the cutter in little pile of flour as you go to prevent sticking.
Bake for 6-8 minutes, until just barely light-golden, then transfer immediately to cooling rack.
Frosting
Melt butter, then stir in milk and vanilla
Add powdered sugar, about a cup at a time, until fairly thick but still spreadable
If desired, divide frosting into smaller bowls and mix in various colors for decorating