Combine the raspberries, sugar, orange juice, and water in a saucepan.
Bring to a boil over medium-high heat, then lower the temperature to medium.
Cook the raspberries, stirring occasionally, until softened and broken up into the sauce, around 10 minutes.
Remove the sauce from the heat and stir in the vanilla extract.
To remove the seeds press through a fine mesh strainer or cheesecloth and then discard the seeds.
Combine the cornstarch slurry ingredients in a small measuring cup, whisking until combined completely.
Place the saucepan back on the stove and whisk the slurry into the mixture rapidly, mixing it in completely to ensure that there are no lumps.
Cook on high heat, whisking constantly until the sauce thickens, around 2-3 minutes.
Remove from the heat and stir in the salted butter until completely melted. Cool until slightly warm, then pour the sauce into a lidded jar, place the lid on and refrigerate.
Refrigerate until needed.
Notes
Yield will depend on how seedy your berries are and how much you boil it down