Start by rinsing the fresh cranberries under cold water and drain them well.
In a large saucepan, combine the cleaned cranberries and 2 cups of sugar. Stir the mixture continuously over medium heat until the sugar completely dissolves.
In a separate small bowl, combine 4 teaspoons of cornstarch with 1 cup of water until it forms a slurry. Gradually pour this slurry into the cranberry and sugar mixture, stirring continuously to avoid lumps.
Increase the heat to bring the mixture to a boil. Continue to stir as the mixture thickens. The cranberries will pop and release their juices.
Simmer for 30 minutes, stirring occasionally, breaking up the cranberries.
Remove the saucepan from the heat. Stir in 1/4 teaspoon of vanilla paste for added flavor.
Allow the mixture to cool slightly before transferring it to a blender. Blend the cranberry filling until it reaches a smooth consistency.
Transfer the blended cranberry filling back to the saucepan or to a heatproof container. Allow the filling to cool completely before using it in the 4-Layer Cranberry Chocolate Tarte. The filling will continue to thicken as it cools.
Pour the filling over the graham cracker crust and transfer to the freezer for about 15 minutes. Do not freeze until frozen, this step is used only to expedite the recipe. Once the cranberries have set and resemble a jelly remove from freezer.