In the bowl of your stand mixer fitted with the paddle attachment, beat the sugar, butter and egg on medium speed for 5 minutes. Add in the grated orange zest.
¾ cup sugar, ⅓ cup butter, softened, 1 egg, 1 tbsp grated orange zest
Meanwhile, whisk together the flour, baking powder and salt.
1½ cups flour, 2 tsp baking powder, ½ tsp salt
With the mixer speed set to low add ½ the flour and then add ½ the orange juice. Mix until well combined and then add the rest of the flour, and the rest of the orange juice. When well combined (you see no more white flour) turn off the mixer.
½ cup orange juice
Toss your berries with 1 tablespoon of flour to coat. Using a rubber spatula gently fold the berries into the cake batter until evenly incorporated.
1 tbsp flour, 1½ cups mixed berries
Transfer the batter to your prepared cake pan and use the back of a spoon to spread it evenly.