Preheat oven to 375 degrees. Line a 9x13-inch metal baking pan with parchment paper, set aside.
In the bowl of a stand mixer, cream sugar and butter until light and fluffy, about 2 minutes on medium-high. Add vanilla extract, almond extract and the whole egg. Mix, then add the egg white and mix until combined.
In a small mixing bowl whisk together cake flour, all-purpose flour, baking powder, cream of tartar and salt. With mixing speed on low, gradually add dry ingredients until no flour pockets remain.
Press into the bottom of the prepared baking pan. Make sure it is a smooth, even layer of dough.
Bake for 15-18 minutes or until firm. Remove from the oven and let cool in the pan for 30 minutes or until no longer warm (note: if the bars are still warm, try popping them into the freezer for 10 minutes before frosting them).
Meanwhile, to make the frosting, in the bowl of a stand mixer fitted with the whisk attachment, cream butter until it becomes light and smooth, about 2 minutes. With mixing speed on low, gradually add the powdered sugar, then the heavy cream, vanilla, and salt. Bring mixing speed up to medium and beat until fluffy, about 1 minute.
Spread over the cooled bars. Top with sprinkles, if desired. To cut clean, crisp bars, refrigerate for 30 minutes before removing the bars from the pan and setting them onto a large cutting board. Remove the parchment from underneath them. Cut using a long, sharp knife.
Notes
-You can add food coloring to the frosting if desired. I prefer using AmeriColor, just a drop gives the perfect tint without being overwhelming.