• Preheat oven to 350 F.
• In a medium bowl, mix flour, baking powder, baking soda, and salt.
• In a large bowl, cream butter and sugar with a mixer on medium speed until light and fluffy.
• Switch the mixer to high speed. Add the egg and vanilla. Then add the sour cream.
• Switch the mixer to low. Add the flour mixture until just combined.
• Using a 1/4 cup measuring cup, drop 6 mounds of dough on a lined baking sheet. You will have to do multiple batches. Bake for 16-18 minutes. Cool on baking sheet for 5 minutes before transferring to a wire rack.

Icing

• In a small bowl, mix 3 cups confectioners' sugar, 6 tablespoons of milk, corn syrup, vanilla, and salt. Sally's recipe calls for 5 1/2 cups but I found that to be too much. Start with 3 cups and you can always add more 1/4 cup at a time. The icing should be thin and spreadable but not extremely watery/drippy.
• Transfer half of the icing to a different bowl. Add 1 tbs of milk and 3 tbs of cocoa powder. Adjust consistency with more powdered sugar if needed.

Assembly

• After cookies have cooled, flip them over to ice the underside. Spread vanilla icing on one side. Let set for 15 minutes. Spread chocolate icing on the other side. Let sit for 15 minutes so icing can set. You can also let it set in the fridge.