PREP: Lightly spritz a 9x13-inch pan with cooking spray (this helps the parchment paper stick nicely), then lay on parchment paper, leaving an overhang for easy removal of the bars. Lightly spray parchment paper with cooking spray and set aside. Melt the butter in the microwave and then set aside for about 5-7 minutes to cool to room temperature. (If it is hot it will melt the sugar and cause greasy cookie bars.) Preheat the oven to 325 degrees F (162 degrees C).
WET INGREDIENTS: Once butter is cooled to room temperature, add in the white sugar. Vigorously whisk until sugar and butter are incorporated, about 2 minutes. (It may seem like the two won't incorporate, but whisk until a smooth mixture forms.) Add in one egg and whisk until combined. Next, add in the egg yolk and vanilla. Whisk until smooth. If using an optional flavoring add in here: either add 1/2 teaspoon pure almond extract (what we typically stick with) or the almond extract and lemon zest, or the clementines zest and 1/4 teaspoon ground cardamom. Whisk to combine.
DRY INGREDIENTS: Add in the baking powder, salt, and correctly measured flour. Mix until just combined, being careful to not overmix (which will result in dense bars). Use a spatula to transfer every bit of this dough to the prepared pan. It's fairly sticky so lightly spritz your hands with cooking spray and then press the dough firmly into an even layer.
BAKE: Bake in the preheated oven for 23-28 minutes or until the top of the bars are no longer glossy and the edges are a very light golden color. Be careful to not overbake; the bars will not be as soft and flavorful. Remove from the oven and let the pan cool on a wire cooling rack until they reach room temperature.
FROSTING: In a large bowl, add the room-temperature butter. Beat on medium speed for 3 minutes, or until completely smooth and fluffy, scraping down the sides with a spatula as needed. Add in the vanilla, cream, and salt; beat to combine. Finally, add in the powdered sugar and then beat the mixture (on low speed) to combine. Once combined, increase the speed of the mixer and beat until the frosting is very light and fluffy-- about 3-4 minutes. It should turn a brighter white. Add a few drops of food coloring (dye frosting to your desired color intensity) Beat on low speed for about 15-20 seconds, just to color the frosting. Use a table knife to spread on the completely cooled bars. Immediately top with sprinkles if using. Use the parchment paper overhang to remove the bars before frosting. Cut into bars and enjoy.
STORAGE: Sugar Cookie Bars are best enjoyed 1-2 days after being made. Leftover bars can be tightly wrapped and frozen but they do lose some taste/texture after being frozen and thawed.