Make sure the butter is softened to room temperature.
Mix all the ingredients together in a bowl until homogenous.
Heat the milk over medium heat in a small saucepan until it's steaming, but make sure it doesn't bubble over or burn. **
In a bowl, whisk together the egg and sugar until blanched (it will turn white as air is incorporated)
Add the cornstarch and vanilla bean paste (if using) and again whisk together until homogenous.
When the milk is hot, slowly drizzle it over the egg and sugar mixture while whisking the whole time.
Once all the milk is in the bowl, pour all the contents back into the saucepan.
Heat over medium heat while whisking the whole time until the mixture thickens (usually 2-3 minutes but could be more). Nothing will happen, and then it will thicken very quickly.
Take off heat and mix in the butter.
Allow the pastry cream to cool for a few minutes and then combine the almond cream and pastry cream.
