Preheat oven to 350 degrees. Butter and flour standard muffin tins. Whisk together flour, baking powder, and salt in a small bowl. Warm milk and butter in a saucepan over low heat.
Beat eggs and sugar with a mixer on high speed until thick and pale, about 5 minutes. Beat in dry ingredients.
Bring milk and butter to a boil. With mixer on low speed, add milk mixture to batter, and beat until smooth. Add vanilla. Divide batter among muffin cups, filling each halfway. Bake cupcakes until light gold, about 15 minutes. Let cool in tins for 10 minutes, then transfer to wire racks. Let cool.
Using a serrated knife, cut each in half horizontally. Spread 1 tablespoon vanilla cream on each cupcake bottom. Sandwich with top. Spoon glaze over each, and serve immediately.
For Vanilla Cream:
Whisk yolks until smooth. Mix sugar, cornstarch, and salt in a medium saucepan over medium heat. Add milk in a slow, steady stream. Cook, stirring, until mixture begins to bubble and thicken, about 5 minutes.
Pour 1/3 of milk mixture into yolks, whisking constantly. Return mixture to saucepan, and cook over medium heat, stirring often, until thick, 2 to 4 minutes. Stir in vanilla.
Pass vanilla cream through a fine sieve into a bowl. Press plastic wrap directly on surface. Refrigerate until cold, at least 1 hour, or up to 2 days.
For Chocolate Ganache Glaze:.
Bring cream to a boil in a small saucepan. Remove from heat, and add chocolate and corn syrup. Let stand for 5 minutes. Whisk until smooth. Transfer to a bowl, and let cool, stirring often. Use immediately.