Line a quarter-sheet baking pan with plastic wrap, making sure the sides of the pans are covered. Set aside.
Pour coconut milk and 1 cup milk into a large heavy bottomed sauce pot. Whisk together and place of medium heat. Stir occasionally to ensure milk does not burn along the sides of bottom of pot. Bring to a simmer.
Meanwhile, whisk together sugar and cornstarch in a medium bowl. Add remaining ½ cup milk and egg yolks. Whisk until smooth.
Once milk is at a simmer, reduce heat. Slowly add about ⅓ cup of hot milk mixture into egg mixture. Whisk constantly to distribute heat. Continue to add milk in intervals until only about ⅓ cup of milk remains in sauce pot.
Pour all of the egg-milk mixture back into the sauce pot. Increase to medium heat. Stir often to prevent mixture from burning along sides of pot. Stir until mixture has thickened to custard like consistency. Allow mixture to come to a boil for 15 seconds to cook out any cornstarch flavor. Remove from heat.
Whisk in butter until smooth. Add in salt and vanilla. Stir until combined.
Pour hot pastry cream into prepared plastic wrapped sheet pan. Spread evenly. Directly cover surface of pastry cream with plastic wrap. Freeze for 10-15 minutes until pastry cream has cooled to room temperature. Remove from the freezer and transfer pastry cream to an airtight container. Keep chilled in the fridge until ready to use.