Add the raspberries and cornstarch to the bowl of a food processor and process until smooth.
Transfer to a medium saucepan and add the sugar and lemon juice. Bring to a boil and let it boil for 3 minutes, stirring occasionally. Remove from heat and stir in vanilla extract.
Press the warm sauce through a fine mesh strainer to remove the seeds.
The sauce will thicken slightly as it cools but is still liquid and perfect for drizzling over your favorite desserts.