Whisk the cornstarch and water together until all the cornstarch has dissolved. (I just use a fork to mix—very easy.) Combine cornstarch mixture, raspberries, granulated sugar, and lemon juice in a small saucepan over medium heat. Using a silicone spatula, stir the mixture, lightly mashing the raspberries as they begin to heat.

Bring to a boil and let it boil for 3 full minutes, stirring occasionally. Remove the pan from heat and—if desired for a richer flavor—stir in vanilla extract.

Press the warm sauce through a fine mesh strainer to remove the seeds, if desired. I use the back of a spoon to press the liquid through the strainer, held over a bowl. It takes a couple minutes to really squeeze it all out.

Feel free to serve warm over warm desserts, but it should be cooled to really thicken up. Cool the sauce completely at room temperature or in the refrigerator. The sauce will thicken slightly as it cools, but the strained version is still liquid and perfect for drizzling.

Cover and store for up to 1 week in the refrigerator.