Preheat the oven to 350ºF and line a 9 by 13 metal pan with parchment paper.

To the bowl of a stand mixer using the paddle attachment (or using a hand mixer), beat the butter and sugar together about 1 minute until creamy. Add the egg and sourdough discard and whip again on medium-high speed until completely combined and the mixture looks light and fluffy. Add in the vanilla extract (or vanilla bean paste) and mix again. Scrape down the sides of the bowl as needed and mix again until completely incorporated.

To a small bowl whisk together flour, baking powder and salt. Add the dry ingredients into the bowl with the butter mixture and beat in the flour mixture until completely combined and a dough forms.

Dump the dough out into the lined baking pan and press into one even layer over the entire pan.

Bake sugar cookie bars for about 35 minutes until the edges are lightly brown and the cookie bars are baked through. I don't like to let the bars get too brown on top, just the edges, or else they can be dry and crumbly. Pull the bars out of the oven and let cool completely.

While the bars are cooling, use a mixer to whip softened butter until creamy, about 1 minute. Add the powdered sugar, heavy cream, vanilla and salt. Whip together for about 3-4 minutes, scraping down the sides of the bowl as needed, until light, fluffy and creamy. Spread on the completely cooled sugar cookie bars. Top with sprinkles if desired and enjoy!Note: Add ½ teaspoon almond extract to the frosting for an extra delicious flavor. This makes a thin layer of frosting. If you want it thicker, double the recipe.