Preheat oven to 375 degrees.* Thoroughly butter and flour a tube, angel food, or bundt pan. 

In a medium bowl, whisk flour, baking powder, and 1/4 tsp salt (reserve the remainder of the salt). 

In a separate bowl or stand mixer, whip the remaining 1/4 tsp salt with the egg whites until the whites are just past the soft peak stage (do not over-whip or whites will become puffy, grainy, and watery). Set aside. 

In the bowl of a stand mixer, add the lemon zest and sugar and mix until combined (if you’ve used the stand mixer to whip the egg whites, no need to clean it first). Add the 6 egg yolks and beat until the mixture is creamy and well-combined, about 3 minutes. Scrape down the bowl and add the lemon juice, lemon extract, vanilla, and oil. Beat 3 minutes more, then scrape the bowl again. Stir in the flour mixture just until all the flour is incorporated. 

If your egg whites are looking droopy, whisk them for a few moments to renew their texture. Add a big dollop to the thick batter and stir it in gently to loosen the mixture, then fold in the remainder of the whites gently and with extreme care to ensure they don’t deflate. Carefully fold in the blackberries. 

Pour batter into the prepared pan and place in the oven. Immediately reduce the temperature from 375 to 350 degrees* and bake undisturbed (resisting any urges to open the oven door for a peek) for 45-50 minutes, until the top is evenly brown and a cake tester comes out clean. 

Cool the cake in the pan on a rack for 5 minutes, then run a sharp knife carefully around the edges of the pan to release the sides. Invert the cake onto a rack, remove the pan, and continue to cool. When the cake is completely cool, dust lightly with powdered sugar if desired. 

Wrap leftovers in plastic or keep in an airtight container.