To make pastry cream, mix 1/4 cup sugar and cornstarch in a bowl until smooth. Add egg yolks and mix until a paste is formed. Stir in 1/2 cup milk.
Combine remaining 1 1/2 cups milk and 1/4 cup sugar in a saucepan and bring to a boil. Pour hot mixture into a bowl, whisking constantly. Pour back into pan.
Cook over moderate heat, stirring constantly, until smooth and thick. Remove from heat and stir an additional minute. Stir in vanilla and transfer to a bowl. Cover with buttered parchment paper touching top and chill a minimum of 2 hours or as long as 2 days.
To make cupcakes, butter and flour a 12-cup and a 6-cup muffin tin.
Melt chocolate in the top of a double boiler or in a bowl over simmering water. In another bowl, combine brown sugar, 1/2 cup milk and 2 egg yolks. Whisk until combined. Add to melted chocolate and stir constantly (while cooking over simmering water) until mixture is shiny and thick, about 3 minutes. Set aside to cool.
In a clean bowl, cream butter and sugar until light. Add remaining 2 egg yolks, 1 at a time, beating well after each addition. In another bowl, mix together, flour, salt and baking soda.
Combine 1/4 cup whipping cream, vanilla and remaining 1/2 cup milk in a small bowl and reserve.
Pour cooled chocolate mixture into creamed butter and sugar. Whisk until smooth. Add combined dry ingredients and cream mixture in three stages, alternating liquid and dry ingredients, and ending up with liquid. Beat egg whites until soft peaks form. Gently fold, all at once, into batter. Spoon batter into muffin cups, about 2/3 full.
Bake in a preheated 325 degrees F oven for 20-25 minutes, until a toothpick inserted in center comes out clean. Set aside to cool, in pan on rack, about 10 minutes. Invert and set aside on a sheet pan lined with parchment or wax paper to cool, about an hour.
To prepare ganache, place chocolate in a medium bowl. In a small saucepan bring 3/4 cup whipping cream to a boil. Pour into chocolate and stir until chocolate is completely melted. Let cool until mixture is less than body temperature.
To make pure white frosting, mix powdered sugar and milk in a small bowl until smooth.
Using the tip of a small paring knife, cut a small cone from bottom of each cupcake. Reserve cones. Scoop out about 2 teaspoons cake from center of each cupcake ( A grapefruit spoon works well for this step) Fill a pastry bag fitted with a plain tip with pastry cream. Pipe cream into cupcakes, then replace reserved cones. Place bottom-side down on the lined sheet pan and chill.
When ganache is room temperature, dip cupcakes in to coat tops. Chill for a few minutes to firm chocolate. Fill a plain-tipped pastry bag with frosting and decorate with a squiggle across each top, trying for 7 loops on each cupcake. Store in refrigerator until serving time.