1In 3-quart heavy saucepan, stir together milk, sugar, flour and egg yolks with whisk. Cook over medium heat 10 to 12 minutes, stirring constantly, until thickened. Remove from heat; stir in 2 teaspoons vanilla. Transfer to bowl; press plastic wrap on surface of custard. Refrigerate until ready to assemble.
2Heat oven to 325°F. Place paper baking cup in each of 24 regular-size muffin cups. In large bowl, beat cake mix and water with electric mixer on low speed 30 seconds, then on medium speed 1 minute. Stir in 2 teaspoons vanilla. Spoon batter into muffin cups, filling completely full.
3Bake 20 to 25 minutes or until golden brown and cracks in tops of cupcakes appear dry. Cool 10 minutes; remove from pans to cooling racks. Cool completely, about 30 minutes.
4Scoop out centers of cupcakes, leaving 1/2-inch border on bottom and side. Spoon or pipe custard into centers of cupcakes. Top each with strawberry half. Store covered in refrigerator.