Into a pot pour frozen or fresh raspberries (12 oz.) and water (1/4 cup). Heat on medium heat until very soft, about 10 minutes. Remove from the heat and break up the mixture with an immersion blender.
To remove seeds strain mixture through a fine mesh sieve. To further extract juice you can scrape out seeds onto a clean cheese cloth. Form a pouch and twist it over a bowl with strained juice. Let it drip and keep twisting the pouch, then discard the seeds. Hand wash the cheese cloth and allow it to dry before putting it away.You should have about 1 cup of fruit puree.
Cake Filling
Measure fruit puree (1cup). Pour fruit puree it into a small pot. Pour in granulated sugar (1/4 to 1/3 cup), freshly squeezed lemon juice( juice from one medium to large size lemon, strained), lemon zest (1tsbp) and corn starch ( 2 and 1/2 tbsp). Whisk well to combine and till no longer white streaks remain. Taste and if needed add more sugar.TIP : Ff you are few tablespoons shy of 1 cup of fruit puree you can top it off with water or other juice (apple juice, strawberry for example.)
Cook mixture on medium heat. Mixture as it cooks will thicken and deepens in color. Bring it to boil and then reduce to slow boil. Stirring constantly cook for about 2 minutes.
Remove from the heat and pour into a bowl. Cover with a food wrap to prevent crust from forming. Allow to cool to room temperature before using.TIP : To speed up the cooling you can cool raspberry filling in a cold water. Pour cold water into a large bowl and carefully submerge small bowl with raspberry filling into the water. Taking care not to let the water overflow into the filling.
Cooled fruit filling can be stored in the fridge at least a week an frozen for 2-3 weeks.
Notes
CAN I USED OTHER FRUIT?
Absolutely. I frequently make strawberry and blueberry filling. Blueberry filling is excellent in my Blueberry Babka.