Cardamom cupcakes with vanilla custard filling and marshmallow frosting

Original recipe from Bluebonnets and Brownies
Oven temp = 350

Cardamom cupcakes (Schoolbread):
3 cups flour
2 Tbs cornstarch
1 Tbs baking powder
1/2 tsp salt
1 Tbs ground cardamom
1 cup (2 sticks) unsalted butter, room temperature
1 3/4 cups sugar
4 large eggs, room temperature
1 Tbs pure vanilla extract
1 1/4 cups milk

Cream butter and sugar. Add eggs, one at a time and mixing well between each addition. Mix in vanilla extract, salt, cardamom and baking powder. Add flour and cornstarch, one cup at a time, alternating with adding the milk. Bake at 350 deg for about 25 minutes.

When cupcakes are cool, cut a little cone out of the top to make room for the vanilla custard.

Vanilla custard:
1 2/3 cups milk
1/4 cup sugar
1/4 tsp vanilla extract
4 egg yolks
1 tablespoon corn starch

In a small pot, bring the milk to a boil and set aside. In a mixing bowl, whisk sugar, vanilla extract and egg yolks until the mixture doubles in volume. Slowly add the hot milk while mixing. Move the custard mixture to the top of a double boiler. Dissolve the cornstarch in a little bit of water and slowly add the cornstarch into the custard. Whisk the custard over the heat until it reaches the desired consistency.

Marshmallow frosting (Martha Stewart's 7 minute icing):
1 1/2 cups plus 2 tablespoons sugar
2/3 cup water
2 Tbs light corn syrup
6 large egg whites, room temperature

Combine 1 1/2 cups sugar, water and corn syrup in a small pot and boil until the mixture reaches 230 deg. At the same time, beat the egg whites in a mixing bowl until soft peaks form. After the sugar mixture reaches 230 deg slowly pour it into the egg whites, while mixing. Whisk the mixture until it is cool and stiff peaks have formed. This should be about 7 minutes.