For Base:2 cups (about 10 ounces) all-purpose flour1/2 teaspoon baking powder1/2 teaspoon kosher salt3/4 cup unsalted butter, at room temperature1 cup (about 7 ounces) sugar1 large egg3/4 teaspoon vanilla extractFor Topping:1 1/2 cups (about 10 1/2 ounces) sugar1 cup heavy cream6 tablespoons unsalted butter, roughly cut into pieces1 teaspoon vanilla1/2 teaspoon salt2 cups salted roasted cashews
Adjust oven rack to middle position and preheat oven to 375°F. Line a 13- by 9-inch baking pan with foil. Grease foil with butter. In a small bowl, combine flour, baking powder, and salt; set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat together butter and sugar until fluffy and light in color, about 4-5 minutes. Beat in egg, then vanilla. Beat in the flour mixture in two additions, scraping down the sides of the bowl each time.
Place dough in prepared pan. Using either floured hands or a piece of plastic wrap, press dough evenly into the bottom of the greased baking pan. Let chill in refrigerator for 20 minutes.
Bake crust until golden brown, about 30 minutes. Remove crust, but keep oven on. Let crust cool completely. While crust is cooling, make filling.
Place sugar in a nonstick saucepan over medium high heat. Let stand until bottom layer of sugar begins to melt, then start stirring, continuing to stir until sugar has turned light brown in color and smooth in texture. Pour in heavy cream, being careful of it splattering. Stir constantly until mixture is smooth again, about 5 minutes. Turn down heat and stir in butter, vanilla, and salt. Take mixture off heat and stir in cashews. Pour mixture into pan. Bake for 15 minutes.
Let mixture cool for at least two hours or until set. When firm to the touch, cut into bars.