Combine sugar and cornstarch in small saucepan and whisk until combined.

Add raspberries, water, and lemon juice and stir together until all ingredients are combined.

Turn stovetop heat to medium (Important! don’t increase the heat above this, you want everything to heat slowly so the sugar dissolves and the mixture thickens properly) and cook, stirring almost constantly, until berries release their juices and mixture begins to bubble (it usually takes about 7 minutes for my mixture to begin to bubble).

Continue to cook, stirring constantly, until thickened (this usually takes me about 2 minutes). Remember that the filling will continue to thicken as it cools, you don’t want it to become pasty in the saucepan or it will be too thick after cooling.

Transfer to a heatproof container and allow to cool to room temperature before spreading over cake. If not using immediately, store in an airtight container in the refrigerator for up to a week. If mixture is too thick to spread over cake after refrigerating, briefly heat it in the microwave for several seconds until it is a spreadable consistency.