Strawberry Compote Filling for Cakes

1 lb. of washed strawberries can also use frozen strawberries or any other type of berry
⅓ cup granulated sugar to taste - may need to add more or less depending on ripeness of your strawberries and personal preference
Zest of half a lemon
2 tablespoons cornstarch, mixed in 3 tablespoons water in small, separate bowl

Add strawberries, sugar, and lemon zest into your saucepan
Mash strawberries (optional) - you can blend your strawberries or mash them with a fork, depending on how smooth or chunky you want your strawberry compote. (I like to keep mine a little chunky, so I lightly mash with a fork.)
Heat saucepan on medium to allow to come to a simmer. Stirring occassionally and scraping down sides, continue to cook until the bubbling slows down and most of the water has evaporated (it does not have to be completely dry).
Mix in the cornstarch-water slurry.
Heat until cornstarch has thickened, about 2 minutes.
Transfer compote to a heat-safe bowl and let cool. Once warm (not hot), refrigerate until completely cold before using compote between your cake layers. Note the cornstarch will continue to thicken as it cools.

Pipe a dam of buttercream around the edge of each cake layer.
Add the compote to the center of the cake and gently spread towards the buttercream dam. Do NOT overfill filling between each layer -- you can always offer extra filling on the side at serving time rather than overfill the cake to avoid risk of leaking or destabilizing the cake.
Make sure the compote is spread evenly flat (get eye-level to check) before adding your next cake layer.

The cook time will depend on whether you are using fresh berries vs frozen, as well as what type of berries you are using since this impacts the water content.