Preheat oven to 350°F. Line a 10x15-inch pan with foil and lightly spray the pan with nonstick cooking spray.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium speed for 2 minutes.

In a medium bowl, combine sugar and lemon zest until the zest is incorporated and the sugar is fragrant. Add lemon sugar to the butter and beat for an additional 2 minutes.

Add eggs one at a time, making sure each egg is incorporated before adding the next. Stir in vanilla extract and mix until thoroughly combined.

In a large bowl, whisk together the flour, salt and baking soda. Gradually add the flour mixture to the butter mixture, mixing until just combined. Add in sprinkles and mix until they are evenly distributed throughout the dough.

Evenly press the dough into the prepared pan. Bake in preheated oven for 30 minutes. Remove pan from oven and place pan onto a cooling rack to cool completely.

Once bars have cooled, prepare the buttercream frosting.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter on medium speed for 4-5 minutes.

Turn mixer to low and gradually add in the confectioners’ sugar until incorporated.

Once the sugar is fully incorporated, turn mixer back to medium speed and beat for an additional 4 minutes. Stir in vanilla extract and 1 tablespoon of milk and mix until combined.

Pipe or spread the buttercream onto Funfetti sugar cookie bars. If desired, immediately top with additional sprinkles.