Combine milks in a medium saucepan. Bring to a simmer over medium heat, about 4 minutes. Remove from heat; add vanilla bean paste, stirring with a whisk until blended.
Combine egg, egg yolks, and sugar in a medium bowl, stirring with a whisk until blended. Gradually add hot milk, stirring vigorously with a whisk. Pour egg mixture through a sieve into a bowl.
Place 4 metal canning jar bands in bottom of a 6-quart oval electric slow cooker. Ladle egg mixture evenly into 4 (8-ounce) ramekins. Cover ramekins with foil. Set 1 ramekin on each band, making sure ramekins do not touch each other or sides of slow cooker. Carefully pour hot water into slow cooker to depth of 1 inch up sides of ramekins.
Cover and cook on HIGH for 1 hour and 45 minutes or until a knife inserted in center of custards comes out clean. Remove ramekins from slow cooker, and cool on a wire rack. Serve warm or chilled.