Preheat oven to 350°F. Lightly spray a 9 x 13″ pan with non-stick spray. Set aside.
In a mixing bowl, combine the cake mix, eggs, water, and vegetable oil. Mix on low speed for 30-60 seconds, using a hand or stand mixer, then scrape down the sides of the bowl. Continue mixing for 1-2 more minutes until the batter is smooth.
Pour the batter into the prepared pan. Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean. Allow to cool completely at room temperature.
For the cream cheese custard: In a clean mixing bowl, combine the softened cream cheese, milk, and vanilla extract. Beat on medium speed until smooth, occasionally scraping down the sides. Add sour cream and continue to beat. Gradually add powdered sugar and beat until smooth. The mixture should be slightly thick. Refrigerate until ready to assemble.
Once cooled, cut the cake into 1″ squares. In clear wine glasses or a large trifle bowl, layer 4-5 cake squares at the bottom, top with 2-3 tablespoons of custard, and repeat layering until all components are used.
Garnish the parfaits with extra red velvet crumbs, red sprinkles, or chocolate chips. Refrigerate until serving.