Step 1

Place 3 sticks unsalted butter and 1 large egg on the counter. Measure out 1/2 cup sour cream. Let everything sit at room temperature until they come to room temperature.

Step 2

Arrange a rack in the middle of the oven and heat the oven to 350ºF. Line the bottom and two opposite sides of a 9x9-inch baking pan with a sheet of parchment paper long enough to hang over the sides by 1 inch to form a sling.

Step 3

Place 2 sticks of the unsalted butter and 1 1/2 cups granulated sugar in the bowl of a stand mixer. (Alternatively, use an electric hand mixer and a large bowl). Beat with the paddle attachment on medium speed until fluffy and lightened in color, about 3 minutes.

Step 4

Stop the mixer and scrape down the sides of the bowl with a flexible spatula. Add the sour cream, egg, 1 tablespoon lemon juice, 2 teaspoons vanilla extract, and 1/2 teaspoon kosher salt. Beat on medium speed until well-combined, about 1 minute (the mixture might look slightly curdled).

Step 5

Stop the mixer and scrape down the sides of the bowl. Add 2 1/2 cups all-purpose flour, and beat on low speed until just fully incorporated. Transfer to the baking pan and spread it into an even layer.

Step 6

Bake until golden-brown around the edges and lightly golden across the top, 30 to 35 minutes. Meanwhile, wash and dry the mixer bowl and paddle attachment.

Step 7

Place the pan on a wire rack and let cool for 10 minutes. Grasping the excess parchment, lift the slab out of the pan and place on a wire rack. Let cool completely, about 45 minutes. Meanwhile, make the frosting.

Step 8

Place the remaining 1 stick unsalted butter and 1 pinch kosher salt in the mixer bowl. Beat with the paddle attachment on medium speed until lightened in color and creamy, scraping down the sides of the bowl as needed, about 2 minutes total. Add 1 teaspoon vanilla extract and beat until combined.

Step 9

Turn the mixer to low speed. Add 2 cups powdered sugar in two additions and beat until completely incorporated. Add 1 to 2 drops food coloring gel or liquid if desired and beat until evenly colored, stopping to scrape down the sides of the bowl as needed.

Step 10

Transfer the frosting onto the cookie slab and spread it into an even layer with an offset spatula. Top with 1/4 cup sprinkles if desired. Let sit at room temperature for at least 30 minutes to let the frosting stiffen. For the cleanest slices, refrigerate the assembled bars for 20 minutes before slicing.

Step 11

Grasping the excess parchment, transfer the slab onto a cutting board. Trim the edges of the slab if desired, then cut into 16 pieces.