Boston Cream Donuts

Name: Boston Cream Donuts
Prep time: 30 min
Cook time: 20 min
Total time: 50 min
Cuisine: American
Ingredients: 16 ounces all-purpose flour; 1 package fast-acting dry yeast ((0.25 oz)); 1 ounce milk (lukewarm); 7 ounces water (lukewarm); 3 Tablespoons granulated sugar; 1 large egg (room temperature); 3 ounces unsalted butter (softened, cubed); ⅓ teaspoon baking powder; ½ teaspoon salt; Oil (for frying); 1 cup milk; 2 egg yolks; 1 Tablespoon cornstarch; ¼ cup granulated sugar; 1 Tablespoon unsalted butter; 1 teaspoon vanilla extract; 4 ounces semisweet chocolate chips; ⅓ cup heavy cream