Frosted Sugar Cookie Bars

Preheat oven to 350 degrees F. Line an 8x8 metal pan with parchment paper with overhang on two sides. Spray lightly.
Whisk flour, baking powder, baking soda, and cream of tartar in a medium bowl. Set aside.
1 1/2 cups all-purpose flour, spooned and leveled, 1/2 teaspoon baking powder, 1/4 teaspoon baking soda, 1/4 teaspoon cream of tartar
In a large bowl using a handheld mixer or stand mixer fitted with a paddle attachment, cream the softened butter for about 1 minute on medium speed. Add the sugar and salt and beat for 30 seconds. The mixture will look sandy. This is correct.
3/4 cup granulated sugar, 1/2 teaspoon Diamond Crystal kosher salt, 1/4 cup unsalted butter, softened to room temperature
Add egg, egg yolk, and vanilla. Beat on medium for 1 minute, until the batter is smooth and comes together. Scrape down bowl as needed.
1 large egg plus 1 large egg yolk, 2 teaspoons pure vanilla extract
Add flour mixture and mix on low until just combined. The dough will be thick.
Fold in 1/3 cup M&Ms by hand with a spatula.
Drop dough in small piles across the pan. Press into an even layer reaching all edges. The dough is quite sticky and a little tricky to spread evenly, spray a little oil on your hands or use an offset spatula to help get it into the corners
Bake 20 to 25 minutes, until the edges are golden and the center is set but still pale. Do not overbake. Allow the bars to cool completely on a wire rack at room temperature before frosting.

With a handheld or stand mixer fitted with a paddle attachment, beat the butter on medium speed until creamy, about 2 minutes. Add confectioners’ sugar, cream, vanilla extract, and salt with the mixer running on low. Increase to high speed and beat for 3 full minutes. Add more confectioners’ sugar if the frosting is too thin, or more cream if it's too thick. Beat in food coloring, if using.
6 tablespoons salted butter, 1 1/4 cups powdered sugar, 3/4 teaspoon pure vanilla extract, 1 tablespoon heavy cream, Pinch of salt, Gel food coloring,
Remove cooled bars from the baking pan using the parchment paper overhang on the sides. Frost the bars with frosting, press 1/3 cup M&M's into the and decorate with sprinkles if desired. Cut into squares. Use a very sharp knife and wipe off with a paper towel between each cut to make neat, even cuts.