Preheat oven to 350F. Line an 8-by-8-inch baking pan with aluminum foil, spray with cooking spray; set aside.
Crust – In a large bowl, combine all crust ingredients and using two forks or your hands, cut butter into dry ingredients until evenly distributed and pea-sized lumps and sandy bits form. The softer the butter is, the quicker and easier it is.
Turn mixture out into prepared pan and hard-pack it with a spatula or hands to create an even, uniform, flat crust layer. Bake crust for about 15 minutes, or just until barely set (pan is going back in oven so don’t overbake or crust will be crumbly). While crusts bakes, prepare the filling.
Filling – In a large bowl (same one is fine, just wipe out with a paper towel), add all filling ingredients, and whisk by hand or beat with an electric mixer until smooth and combined; it’s okay filling is on the thin side. I prefer a stronger almond flavor and use 2 teaspoons extract, if you don’t, scale the amount back.
After crust has baked for about 15 minutes, remove pan from the oven, and evenly pour filling mixture over the crust. Filling will likely slide into place, but lightly spread it into corners using a spatula if necessary.
Return pan to oven and bake for about 16 to 19 minutes, or until filling is no longer jiggly in the center and edges have begun to set up; filling will set up more as it cools.
Remove pan from oven, and place on a wire rack to cool completely before frosting.
Frosting – In a large bowl (same one is fine, just wipe out with a paper towel), add the butter, confectioners’ sugar, almond extract, and slowly drizzle in the cream while whisking by hand or beating with an electric mixer. As necessary, add additional cream (or sugar) to reach desired glaze consistency.
Evenly frost the bars, smoothing frosting lightly with a spatula as necessary.
Evenly sprinkle with slivered almonds.
Allow bars to cool in pan uncovered for at least 1 hour (or overnight and cover with a sheet of foil) before slicing and serving so frosting can set up. Bars will keep airtight at room temperature for up to 5 days. I’m comfortable storing frosted items at room temp; store in the fridge if you’re not, noting bars could become overly dry in the fridge, or store in the freezer for up to 6 months.