This raspberry sauce is so beautiful and delicious, you and your Valentine may not even need the cake. This will absolutely work even if you use frozen raspberries, but nothing compares to the flavor of the sauce if you can somehow find and use fresh raspberries.
Combine raspberries, sugar, water, and lemon juice in a saucepan over medium heat; cook and stir until raspberries break down, sugar dissolves, and sauce is heated through, 3 to 7 minutes.
Press sauce through a fine-mesh strainer to remove seeds. Cool to room temperature; cover with plastic wrap and refrigerate until chilled, at least 45 minutes.
Notes
Chef's Note
If you want something a little thicker to serve over ice cream, for example, don't add any water.