Raspberry Cake Filling

• If possible, make the raspberry filling in advance! The extra time will allow the filling to thicken and develop its flavor.
• Place 4 cups frozen or fresh raspberries, 3/4 cup granulated sugar, 1 Tbsp lemon juice, 2 tsp lemon zest, and 1/4 cup of water in a saucepan and heat over medium-high heat, uncovered.
• Stir the mixture until it begins to boil. Lower the heat to medium-low, and allow the filling to simmer for 10-15 minutes.
• Turn off the heat and remove the pot from the stove. Pour the filling into a metal strainer suspended over a medium-sized bowl, and push through using a rubber spatula. Use a decent amount of pressure to really get all the liquid through the sieve. You should be left with about 1/2 cup of seedy pulp.
• In a separate small bowl, make a slurry by combining the remaining 3 Tbsp of water with 3 Tbsp cornstarch. Stir until completely smooth (no lumps)
• Add this cornstarch mixture to the strained raspberry filling, and stir until it is incorporated. It will go from cloudy/milky to clear and glossy as it thickens.
• Rinse out your pot and pour the filling back into it (you don't want any residual seeds getting into the filling).
• Heat over a medium-high heat, and be sure to stir constantly during this stage to prevent the filling from burning.
• Cook until the mixture begins to boil, then reduce the heat to medium-low. Cook 1–3 minutes after it starts bubbling, until it’s thick enough to coat a spatula.
• Turn off the heat and pour the raspberry filling into a separate bowl to cool. Cover with plastic wrap and place in the fridge for at least 1 hour, or preferably overnight.