Place chopped chocolate in a large heatproof bowl.
Place heavy cream into a small saucepan and cook over med heat, stirring often, until it just starts to simmer. Do not boil.
Pour hot cream over chopped chocolate and cover bowl with plastic wrap. Let sit 5 minutes.
Stir gently with a spatula until completely smooth. Stir in softened butter. If needed, place over a double boiler and stir until all chocolate and butter has melted.
Place plastic wrap directly on top of chocolate ganache. Allow to thicken and set overnight at room temperature.*
Pastry Cream (make in advance):
Place 1 1/2 cups milk and vanilla bean paste into a medium saucepan. Cook over high heat, stirring often until it just starts to simmer. Remove from heat.
In a medium bowl, whisk remaining 1/2 cup milk, egg yolks, sugar, and cornstarch.
While whisking vigorously, slowly pour the hot milk into the egg mixture.
Return mixture to saucepan and cook over high heat, whisking constantly, until it thickens and comes to a boil.
Pass through a strainer and into a small bowl. Place plastic wrap directly on top of custard to prevent a skin from forming. Cool to room temperature then place in refrigerator a couple hours to set.
Once set, whisk well by hand to smooth out completely.
Prepare as per recipe instructions.
Place one layer of cake on a cake stand or serving plate. Using Wilton tip 6B, pipe a border of rosettes around the perimeter of the cake layer. Refrigerate for 20mins to set.
Meanwhile, using various sizes of French and round tips, pipe different sized dollops of ganache and pastry cream over the remaining layer. I recommend flipping the layer upside-down and piping on the "bottom" flat surface. I used: French tips (Wilton 6B, Wilton 32, Ateco 867) for the ganacheRound tips (Wilton 1A & 2A) for the pastry cream
Remove other layer from the fridge, and carefully fill the center with pastry cream. I used a piping bag and did dollops to fill in the center since the bags were already filled.
Gently place one layer on top of the other. Alternatively, you can wait till the bottom layer is chilled and filled before stacking and decorating the top layer.
* You can speed up the thickening of the ganache by placing the it in the fridge, but you need to stir it often. Do not leave it in the fridge overnight as it will harden.
Liv for Cake - https://livforcake.com/boston-cream-pie/