1stick (4 ounces) unsalted butter, at room temperature
2ouncescream cheese, at room temperature
3/4cupgranulated sugar
1large egg plus 1 egg yolk
1teaspoonvanilla extract
For the chocolate frosting:
1 1/2sticks (6 ounces) unsalted butter, at room temperature
1 1/4cupspowdered sugar, sifted
1/3cupcocoa powder, sifted
3tablespoonsheavy cream
1teaspoonvanilla extract
1/8teaspoonsalt
Rainbow sprinkles
Instructions
To make the bars:
Preheat the oven to 350°F. Line an 8 by 8-inch baking pan with foil, leaving an overhang on all sides.
In a medium bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt.
In a large bowl, use an electric mixer to beat the butter, cream cheese, and sugar on medium-high speed until well combined and smooth, about 3 minutes. Add the egg, egg yolk, and vanilla and beat until combined. On low speed, gradually add the flour mixture and beat until combined.
Press the dough into the prepared pan. Bake for 25 minutes, or until lightly golden brown on top. Place the pan on a wire rack and let cool completely.
To make the frosting:
In a large bowl, use an electric mixer to beat the butter on medium-high speed until creamy, about 2 minutes. On low speed add in the powdered sugar, cocoa powder, cream, vanilla, and salt. Increase speed to high and beat for 3 minutes, or until light and fluffy.
Remove the bars from the pan using the foil overhang. Frost with a thick layer of frosting. Sprinkle with sprinkles before cutting into squares. Serve or store in an airtight container at room temperature for up to 2 days or in the fridge for up to 5 days.