This summer I made a delicious lemon blackberry cake to test both my piping and filling-making skills. I used frozen blackberries from last year’s crop – our blackberry plants bloom very late in the season – and it turned out very well. If you’re using frozen berries, just let them thaw first.
As with raspberry filling, blackberry filling requires some serious sieving time to remove the seeds. Blackberry seeds are larger than raspberry seeds, so you’ll want to clean out your sieve a few times during the straining process. I also recommend working in small batches, rather than trying to press your entire mixture through the sieve at once.
While my original recipe made about 1/3 cup, the recipe below would yield around 1 cup.
- 18 ounces blackberries, mashed
- 2 1/4 cups powdered sugar
- 6 teaspoons cornstarch
- 3 teaspoon lemon juice
In a medium saucepan, stir together blackberries, powdered sugar, cornstarch, and lemon juice. Cook over medium-high heat until mixture boils, then allow to boil for 1 minute, stirring well, until the mixture thickens.
Remove from heat and press through a fine sieve to filter out the seeds. Allow to cool before using.