Warm the whole milk in a pot over low heat and let it simmer gently.
Once it starts to bubble slightly around the edges, add the white vinegar and stir continuously until the milk curdles and separates into curds and whey.
Remove the curds using a slotted spoon and transfer them to a bowl. Cover and let them cool completely.
To make the cream base, whisk the heavy whipping cream, whole milk, and cornstarch in a separate saucepan without turning on the heat. Whisk together well until the cornstarch is fully dissolved and no longer clumpy.
Using a mortar and pestle, crush the mastic with a small pinch of sugar until finely ground. Make sure no large chunks remain.
Place the saucepan over medium-high heat and add the granulated sugar and crushed mastic.
Whisk continuously for about 10 minutes until the mixture thickens to a custard-like consistency. You want to continuously whisk so the bottom doesn't burn.
Once thick, add in the orange blossom water, give it one final stir, and then remove from heat.
Once the cheese curds have cooled, mix them together until well combined.
Transfer the cream mixture to a flat plate and cover with plastic wrap so it doesn't develop a crust. Refrigerate until completely cool.
Your homemade ashta is now ready to be used in pastries and layered desserts or enjoyed on its own! Keep it refrigerated until ready to serve.