Preheat oven to 350°. Spray a standard-size muffin tin with nonstick spray. Beat cake mix, water, oil, and egg whites in a large bowl on low speed until moistened. Beat on medium speed for 2 minutes. Pour batter into cups, filling halfway. Bake for 18 to 21 minutes or until toothpick inserted in center is clean. Remove pan from oven; cool. Repeat with remaining batter, if necessary.

Whisk together egg yolks, sugar, and cornstarch in a medium bowl; set aside. Slice vanilla bean lengthwise; scrape seeds into medium saucepan. Add bean pod, heavy whipping cream, and milk. Cook over medium-low heat just until bubbly. Slowly pour hot cream mixture over the egg yolk mixture, whisking constantly until combined. Remove the vanilla bean pod.

Return mixture to saucepan. Cook and whisk over low heat until mixture thickens and reaches 165° (do not boil). Remove from heat; transfer to a bowl set in ice water to cool quickly, stirring occasionally. When cool, cover with plastic wrap; refrigerate 30 minutes.

Transfer custard into center of cupcakes, letting custard spill out of top. Sprinkle cupcake tops with Demerara sugar. Repeat for a thicker layer, if desired. Cool before serving.