Preheat the oven to 350. Line a cookie sheet with parchment paper and set to the side.
In a medium-sized bowl, combine the flour, baking powder, and salt. Mix to combine.
In a large bowl with a hand mixer or in the bowl of a stand mixer with a paddle attachment, cream the butter for 2-3 minutes, until light and fluffy. Add in the sugar, scraping down the bowl as needed. Add the egg and egg yolk, followed by the extracts. Mix to fully combine and continue to scrape down the bowl as needed.
Alternate adding the sour cream and the flour mixture to the butter mixture. Mix until fully combined, scraping down the sides as needed. The dough will be very thick, like cake batter.
Scope ¼ of the dough on to the sheet of parchment to form the cookies. If needed, use your fingers or a spoon to smooth out the dough to make it in a circular shape. Bake the cookies for 12-14 minutes, until they are golden brown on the bottom.
While the cookies are baking, make the frosting. In a large bowl, combine the powdered sugar, boiling water, heavy cream, corn syrup, vanilla, and salt. Use a whisk to break up the powdered sugar and mix until smooth. Split the icing in half and use blue food dye to color the icing a light blue color.
Take the cookies out of the oven and let them fully cool on a cooling rack. When the cookies are ready, frost half of the cookie with the white base using an offset spatula or a spoon. Let the cookies sit for about 10 minutes before frosting the other side. Frost the other side with the blue frosting when ready.
When the cookies are fully frosted, place them back in the fridge to set for about an hour. Don't rush this process, as this will keep their crisp shine and won't allow for the icing to easily smudge.
