Add your raspberries and sugar to a medium-sized saucepan over medium heat.
Heat your raspberry puree until it begins to bubble. Increase the heat to medium-high heat if needed.
In a small bowl, combine the water and the ClearJel (or cornstarch) and mix until smooth. Don't just add the ClearJel (or cornstarch) directly to the hot liquid or you'll end up with lumps of thickener in your filling.
Add your ClearJel mixture (or cornstarch slurry) to the hot liquid and mix for one more minute to thicken the mixture.
Add in the lemon juice and lemon zest and continue cooking for one more minute.
Transfer the mixture to a heat-proof container and cover it with plastic wrap (so that it's touching the surface) and allow it to cool to room temperature or place it into the refrigerator to cool before using it. Pro tip - If you pour the raspberry filling onto a sheet pan or cake pan and place it into the fridge, it will cool faster.
Using the raspberry filling
Make sure your raspberry filling is fully cooled down before you use it.
Pipe a dam of buttercream around your cake layer and spread a thin layer of your raspberry filling inside. The buttercream keeps the filling from oozing out the sides of the cake.
Keep your raspberry filling layer thin. No more than ¼" thick.
If you want more raspberry flavor, you can mix some of the raspberry filling in with buttercream or even whipped cream. Not only does it make a gorgeous pink color but it tastes freaking delicious.
You can use the leftover raspberry filling for donuts, cupcakes, thumbprint cookies, homemade hand pies, or just freeze the raspberry mixture in an airtight container to use later!
Protip - If you would like seedless raspberry cake filling, then strain your mixture through a fine mesh strainer to remove the seeds, and then return the mixture to the saucepan and bring it back to a bubble.Fresh or frozen raspberries will work for this recipe! If you've got a lot of fresh raspberries to use up, make a big batch of this fruit filling and freeze it to use later. If you don't have fresh raspberries then it's really more cost-efficient to buy them frozen. Bonus: frozen berries are picked at the peak of ripeness and are always very sweet and delicious!Lemon zest and lemon juice will add some brightness and flavor to your raspberry filling that is lost during cooking. Don't worry, your filling won't taste like lemons.ClearJel is similar to cornstarch but stays clear when it's cooked and keeps a smooth consistency even when cooked at high temperatures. If you don't have ClearJel you can substitute with equal parts corn starch.