YIELD: Serves 8
7 ounces fresh blackberries (about 1 1/4 cups; 200g), firm and tart
4 ounces unsalted butter (about 8 tablespoons; 110g), softened to about 60°F (16°C)
5 1/4 ounces sugar (about 3/4 cup; 145g)
1 1/4 teaspoons baking powder
1/4 teaspoon baking soda
1/4 teaspoon (1g) Diamond Crystal kosher salt; for table salt, use about half as much by volume or the same weight
1/8 teaspoon ground cinnamon
2 ounces egg whites (about 1/4 cup; 55g), brought to about 70°F (21°C)
5 1/2 ounces bleached cake flour (about 1 1/4 cups, spooned; 155g) sifted (see note)
1 batch Cream Cheese Frosting
Additional blackberries, to garnish
TO MAKE THE CAKE:
In the bowl of a stand mixer, combine butter, sugar, baking powder, baking soda, salt, and cinnamon.
Mix on low speed to roughly incorporate, then increase to medium and beat until fluffy and light, about 5 minutes. About halfway through, pause to scrape bowl and beater with a flexible spatula
With mixer still running, add egg whites a little at a time, letting each addition fully incorporate before proceeding to the next.
Reduce speed to low and sprinkle in about 1/3 of cake flour, followed by 1/3 of prepared blackberry purée. Repeat with remaining flour and fruit, working in thirds as before.
Scrape bowl and beater with flexible spatula and resume mixing on medium speed for about 3 seconds to ensure everything is well combined. The batter should look bright purple and register between 65 and 68°F (18 and 20°C) on a digital thermometer. (Significant deviation indicates ingredients were too warm or too cold, which can lead to textural problems with the cake.)
If the batter looks streaky or tie-dyed, gently fold with a flexible spatula until streaks disappear.
Scrape into prepared pan, spread into an even layer, and bake until puffed and firm, about 30 minutes.
Cool directly in pan for 1 hour, then run a butter knife around the edges to loosen.
Invert onto a wire rack, peel off parchment, and place on a serving plate until fully cool, about 1 hour.
Top with a batch of Cream Cheese Frosting and a handful of fresh blackberries and cut with a chef’s knife to serve. Wrapped in plastic, this cake will keep for up to 3 days at cool room temperature.
Using a blender, food processor, or immersion blender, purée blackberries until totally smooth.
If seeds bother you, the purée can be strained using a fine-mesh sieve; if you do plan to strain, add a couple extra blackberries to make up for the loss.
Measure out exactly 6 ounces fresh blackberry purée (about 3/4 cup; 170g) and bring to about 70°F (21°C) before using.
Adjust oven rack to lower-middle position and preheat to 350°F (180°C).
Lightly grease an 8- by 2-inch or 8- by 3-inch anodized aluminum cake pan and line with parchment.