Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a bowl, mix the yellow cake mix, melted butter, and egg until well combined. Press this mixture evenly into the prepared dish to form the base.
In a large bowl, beat the cream cheese until smooth. Add the eggs, vanilla, cinnamon, and optional nutmeg. Mix well.
Gradually beat in the powdered sugar and then stir in the melted butter until smooth.
Pour this creamy filling over the cake base, spreading it evenly.
Bake for 40–45 minutes. The edges should be golden, and the center should be set but still a bit gooey. Let it cool completely before adding the buttercream.
In a saucepan, melt the butter and brown sugar over medium heat. Stir until the sugar is dissolved.
Add the heavy cream and vanilla to the melted mixture and let it simmer for 2 minutes. Remove from heat and let cool a little.
Beat the powdered sugar into the toffee mixture until it becomes smooth and creamy.
Pour this delicious buttercream over the cooled cake, spreading it gently over the top.
Allow the buttercream to set for about 15 minutes before slicing and serving.