First, puree the berries and push the puree through a fine mesh strainer to remove the seeds. The straining isn't necessary if you don't mind seeds in your cake, but I preferred the light texture without the crunchy seeds.
Preheat your oven to 350° F. Grease an 8" cake pan and set aside.
In the bowl of a stand mixer, combine butter, sugar, baking powder, baking soda, and salt. Beat until light and fluffy. Scrape down the sides of the bowl as needed.
With the mixer on low, gradually add the egg whites to the bowl and mix to incorporate.
Add ⅓ of the cake flour (see note), followed by ⅓ of the berry puree, allowing the ingredients to incorporate after each addition. Repeat adding the remaining flour and puree ⅓ at a time until all ingredients are added to the bowl. Mix just until incorporated.
Scrape the batter into the prepared pan and bake 30-32 minutes, or until a toothpick inserted near the center comes out clean. Allow cake to cool in the pan for an hour before removing it to a wire rack to cool completely.
For Frosting:
In the bowl of a stand mixer, combine the sugar, cream, and vanilla bean paste (see note). Using the whisk attachment, beat until the mixture is as thick as greek yogurt.
With the mixer running, add the cream cheese a few tablespoons at a time, allowing each addition to incorporate before adding the next. Scrape down the sides of the bowl as needed.
Once all the cream cheese is added, whip on high speed until the frosting is smooth and light in texture, 2-3 minutes.
Frost the cooled cake and top with additional fresh blackberries, if desired.Enjoy!
Notes
Note on cake flour: The cake flour ensures a light, tender cake! If you can't find cake flour in your local grocery store, you can find it here.Note on vanilla bean paste: I love the kick of vanilla flavor (and the beautiful vanilla bean flecks!) from the vanilla bean paste in the frosting! If you're looking for vanilla bean paste, you can find it here. If you'd like, you can substitute with 1 teaspoon of vanilla extract.