Prepare Matcha Filling:
Warm 1 cup milk, 1 cup heavy cream, 1 tablespoon sugar, and vanilla extract in a small saucepan over medium heat until simmering.
Reduce heat and steep 2 bags of Matcha tea in the mixture for 10 minutes over low heat. (Alternatively, you can add loose matcha powder directly to the cream mixture. This provides the filling with a richer green color.)
In a medium bowl, whisk together egg yolks and remaining sugar until thick and creamy. Add cornstarch and continue whisking until fully incorporated.
Remove matcha tea bags from the cream mixture and slowly pour about ½ cup of the warm matcha cream into the bowl with the egg yolk, sugar and cornstarch mixture, whisking continuously. Continue adding cream mixture, 1/2 cup at a time, until incorporated. Pour mixture back into saucepan and cook over medium heat, whisking continuously, until it thickens and comes to a boil. Let boil, still whisking, 2-3 minutes.
Remove from heat, add butter, and stir until melted. Cover with plastic wrap and chill for 1 hour.
Preheat oven to 350 degrees F.
Make a simple syrup – heat ½ cup sugar + ½ cup water in a small saucepan on low heat until sugar is dissolved. Let cool slightly.
Meanwhile, cut desired crostini shape from slices of Martin’s Potato Bread.
Once simple syrup is cooled slightly, use a slotted spoon to quickly “dunk” one side of each crostini in the simple syrup. Note: Be careful not to fully saturate the crostini.
Place crostini on a greased or lined baking sheet with the simple syrup side up. Bake until crostini just start to brown, about 10-15 minutes.
Pipe the matcha filling onto the prepared crostini. Top with berries and powdered sugar, if desired.