Preheat oven to 350?F (175?C).

In a large bowl, use a hand mixer to beat together the cake mix, melted butter, and egg until combined?about 2 minutes. Press into the bottom of the parchment paper-lined 9x13" baking dish.

Wipe out the same bowl. Add the cream cheese, eggs, vanilla, and melted butter and use a hand mixer to beat until smooth. Mix in the powdered sugar, cup by cup, until smooth and creamy.

Add the filling to the baking dish, then add spoonfuls of the pistachio cream all over the top of the filling and swirl gently with a knife.

Place it in the oven and bake for 30?35 minutes, or until the edges are set and starting to turn golden brown but the center is still jiggly.

Let cool and stiffen slightly before cutting into squares.

Sprinkle with flaky salt before serving.