Preheat oven to 325 degrees.
Since the cake batter is heavy, use TWO cupcake liners in EACH cup of a 12-cup muffin tin. If using two muffin tins (6 cups each), be sure to place TWO cupcake liners in EACH cup. After placing two liners in EACH cup of second tin, fill two empty cups with water to prevent warping. Set aside.
In large bowl, combine softened cream cheese (room temperature), 1/2 cup sugar, 4 eggs, and 2 teaspoons vanilla extract. With mixer on low speed, mix together until smooth and creamy (about 1 minute).
Add 1/2 cup melted unsalted butter (or vegetable oil), 1/2 cup of water and cake mix. With mixer on low speed, mix together just until combined (about 2 minutes). Batter will be thick and creamy.
Place 2-1/2 generous tablespoons of cake batter in each cup. Use back of spoon to even batter in cup liner. If the liners are filled to the top with batter, the cupcakes will rise dramatically and will shrink slightly while cooling.
Bake 20-25 minutes OR until top springs back when lightly pressed OR a toothpick inserted in center of each cupcake comes out clean.
Allow to cool before frosting. If frosted while warm or hot, the frosting will melt into a glaze.
NOTE: Although these cupcakes are delicious warm, they taste much better after 24 hours in fridge.