Step 1

Combine raspberries and sugar in a medium saucepan.

Step 2

Cook over medium heat, stirring frequently, until the raspberries release their juices and break down, about 8-10 minutes.

Step 3

In a small, separate bowl, whisk together the cornstarch, lemon juice, and water to create a slurry. Add to the raspberry mixture while stirring and continue to cook another 1-2 minutes until thickened and goes from cloudy to clear. The filling will continue to thicken as it cools, so it shouldn't be too thick and paste-like in the pan.

Step 4

Remove from heat and cool completely before using to fill a cake.