Preheat your oven to 350°F (175°C).
Cream together the unsalted butter and confectioners sugar until light and fluffy. Gradually add in all-purpose flour, cornstarch, and salt (if using). Mix until just combined.
Grease your 9x9-inch baking pan or line it with parchment paper. Press the shortbread mixture evenly into the bottom of the pan. Bake in the preheated oven for about 15-20 minutes, or until lightly golden.
In a separate bowl, beat together the softened cream cheese and granulated sugar until smooth. Add in the egg, all-purpose flour, and almond extract. Mix until well combined.
Pour the cream cheese filling over the cooled shortbread base, spreading it out evenly. Bake for an additional 20-25 minutes, or until the filling is set and lightly browned on top.
Allow the bars to cool completely in the pan on a wire rack. In the meantime, make the frosting by beating together the softened unsalted butter, confectioners sugar, almond extract, and cream or milk until smooth and creamy.
Once the bars are completely cooled, spread the frosting evenly over the top. Sprinkle with slivered almonds for garnish.