01 -
Blend your blackberries until smooth, strain out seeds if you like, and set aside 6 ounces.
02 -
Turn the oven to 350°F, grease an 8-inch pan, and add parchment at the bottom.
03 -
Whip the butter, sugar, and dry mix until fluffy, then slowly mix in the egg whites.
04 -
Alternate between mixing in blackberry puree and flour bit by bit till combined.
05 -
Pour batter into the pan and bake for about 30 minutes or until puffed up.
06 -
Let it sit in the pan for an hour, then remove and let it cool fully.