Boston Cream Cake Roll

Soft, fluffy cake is rolled with creamy homemade custard and topped with a layer of rich chocolate ganache.
Ingredients
Custard:
1 cup whole milk
⅔ cup heavy whipping cream
4 tablespoons unsalted butter
⅔ cup granulated sugar
Pinch of salt
4 large egg yolks
3 tablespoons all-purpose flour
2 tsp cornstarch
1½ teaspoons vanilla extract
Cake:
3 eggs
¾ cup white granulated sugar
⅓ cup water
1 tsp vanilla extract
1 cup all-purpose flour
1 tsp baking powder
Pinch of salt
Ganache:
4 ounces semisweet chocolate, finely chopped
1/3 cup heavy whipping cream, boiling
Instructions
Custard: