Heat oven to 350°F. Line a 9-by-13-inch pan with a strip of parchment, allowing edges to hang over opposite sides. In a large bowl whisk together flour, baking powder and salt.
In another bowl beat together butter and sugar until light and fluffy. Stir in egg, egg white, sour cream and vanilla and mix until blended. Add flour mixture and stir until just combined.
With greased hands, gently press dough into pan. Bake until the edges are lightly golden, 17 to 19 min. Cool completely, then frost and decorate. Use the parchment overhang to lift from pan; slice and serve.
Vanilla Frosting-- Beat together 6 tablespoon softened, unsalted butter, 2½ cups confectioners' sugar and 3 to 4 tablespoon milk until fluffy. Stir in ½ teaspoon vanilla extract and ⅛ teaspoon salt. Tint with food coloring.
Notes
What I will tell you about the technique of a bar cookie, is this: It's very important (only to make your life easier) to line the bottom of the sheet pan with parchment. Cut it so it fits just inside the bottom of the pan and lays flat. Then allow the ends to hang over the pan a bit. This will allow you to remove the sugar cookie bars in one piece to frost and cut easily outside of the pan.