Rinse fresh raspberries under cold water or simply thaw frozen berries (use frozen berries without sauce) *Take Note-Frozen berries will have extra water in them when cooked down so adjust added water to desired consistency.
Add raspberries and sugar in the pot over mediium heat. Cook down for a few minutes. Use a potato masher to press down the berries. This speeds up the reduction process.
Add the water, and lemon juice, stir and bring to a boil. Reduce heat and simmer for approx. 15 minutes stirring continuously with a wire whisk.
Remove from the heat. With a bowl under the strainer scoop the sauce in. Use a silicon spatula to press the sauce through the strainer. This takes several minutes. You'll want to press all the juice down so none is wasted.
Run the spatula under the strainer and scrape the excess sauce into the bowl. Repeat a few times. Do this until all the sauce and juice are pushed through and there is nothing left but seeds.
Mix your 3 tbsp of cornstarch with 3 tbsp cold water in a tiny bowl.
Toss out the seeds/pulp and pour the sauce back into the pot. Under low heat slowly add the cornstarch and water mixture until the sauce thickens. Pour a little at a time to guage the consistency. You many not need all of it. Continue to whisk while thickening.
Once you see a loose jam-like consistency take the sauce off the heat and cool. The sauce continues to thicken when cooling and will be thicker after chilling.
Cool, store in an airtight container, and chill in the frig overnight for the best consistency.
When ready to spread your Heavenly Raspberry Cake Filling on your cake layers simply take it out of the frig, whisk to loosen the filling, and evenly spread (inside your frosting "dam") no more than 1/4" thickness onto each cake layer.