Whisk the egg whites with sugar until they are smooth and thick like whipped cream.

Add one egg yolk at a time and whisk until it mixes properly.

In a separate bowl, mix the flour with baking powder. Then, add olive oil to the bowl and mix it until it's properly incorporated.

Add the flour mixture to the egg mixture and using a rubber spatula, fold it from the center. Continue folding it till everything is mixed properly.

Add the vanilla extract, lemon zest and mix it again.

Next, grease a baking pan with some butter.

Pour the batter in the baking pan and top it with the blackberries.

Bake it for about 45 to 50 mins at 375 F preheated oven.

Garnish it with some powdered sugar and enjoy!