
Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking dish with butter.
In a medium bowl, beat the cream cheese, melted butter, powdered sugar, and kosher salt until smooth and creamy.
Add the egg and mix until fully incorporated.
Spread the cream cheese mixture evenly in the prepared baking dish.
Spoon the lemon pie filling over the cream cheese mixture, spreading it out evenly.
Sprinkle the cake mix evenly over the lemon pie filling. Do not stir — this will form a crumbly topping as it bakes.
Pour the remaining 1 tablespoon of melted butter over the cake mix.
Bake for 45–50 minutes, or until the top is golden and bubbly.
Let cool slightly before serving.
Serve warm with whipped cream or vanilla ice cream, if desired.