Preheat the oven to 350ºF and line a 9x9-inch baking pan with parchment paper or spray with non-stick baking spray.
Using a hand mixer or stand mixer fitted with the paddle attachment, beat the softened butter and sugar together on medium speed until creamy, pale, and fluffy, about 2 minutes. Add the egg and egg yolk and mix on high until combined, about 1 minute. Scrape down the sides and bottom of the bowl as needed. Add the vanilla and almond extract and beat on high until combined, about 1 minute.
Stir together the flour, cornstarch, baking soda, and salt in a medium bowl. Add the dry ingredients to the wet ingredients and mix on low speed until just combined.
Transfer the dough to the prepared baking pan and press/smooth into an even layer. Bake for 30-35 minutes or until lightly golden brown on the edges and a toothpick inserted in the center comes out mostly clean with a few moist crumbs. Do not over-bake. If the bars are browning too much before 25 minutes, tent the baking pan with foil. The bars will slightly puff up in the oven, but settle as they cool.
Allow the bars to cool in the pan set on a wire rack for at least an hour before lifting out. The bars must be completely cooled before frosting.
Make the cream cheese frosting: Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the softened butter and cream cheese on high until creamy, about 1 minute. Add the lemon juice and vanilla and beat on high until combined. Add the powdered sugar and beat on low speed for 1-2 minutes until smooth and fluffy.
Spread the frosting over the cooled sugar cookie bars, garnish with sprinkles, and cut into squares. Cover leftover bars and store in the refrigerator for up to 1 week.