Preheat the oven to 350°F. Grease a 13x9-inch baking dish thoroughly and set it aside.

In a large mixing bowl, combine the white cake mix and toasted chopped pecans. Add the cold cubed butter and cut it into the mixture until coarse crumbs form.

Press the crumb mixture evenly into the bottom of the prepared baking dish, forming a firm crust layer.

In a separate mixing bowl, beat the softened cream cheese, granulated sugar, 1 egg, and 2 tbsp of the sweetened condensed milk until smooth and fully combined. Set the cream cheese mixture aside.

In another bowl, combine the mashed sweet potatoes, the remaining 2 eggs, the remaining sweetened condensed milk, and pumpkin pie spice. Stir until the mixture is smooth and evenly blended.

Pour the sweet potato mixture over the prepared crust and spread it into an even layer.

Drop spoonfuls of the cream cheese mixture over the sweet potato layer. Using a knife, gently swirl the cream cheese mixture into the sweet potato mixture to create a marbled effect.

Bake for approximately 45 minutes, or until the bars are set and the top is slightly golden.

Remove the baking dish from the oven and allow the bars to cool completely at room temperature.

Refrigerate the bars until fully chilled before cutting into squares. Serve chilled.