Blackberry buttermilk cake

Ingredients

• 3/4 cup (1-1/2 sticks) unsalted butter, room temperature, plus more for pan and parchment
• 2-1/3 cups cake flour (sifted, then measured) plus more for pan
• 2-1/2 cups (10 ounces) fresh blackberries
• 1/4 cup plus 1-1/3 cups sugar
• 1-1/2 teaspoons baking powder
• 3/4 teaspoon salt
• 1/2 teaspoon baking soda
• 3 large eggs, room temperature
• 2 teaspoons vanilla extract
• 1-1/2 teaspoons finely grated orange zest
• 1 cup well-shaken buttermilk
• Confectioner' sugar, for dusting