Preheat oven to 350°F (175°C). Line an 8×8″ baking pan with parchment paper, leaving an overhang.,In a bowl, cream together softened butter, granulated sugar, and brown sugar until light and fluffy.,Beat in eggs one at a time, then stir in vanilla extract.,In a separate bowl, whisk together flour, baking powder, salt, and lemon zest if using.,Gently mix dry ingredients into wet just until combined.,Spread about two‑thirds of the batter evenly into the prepared pan.,Scatter blackberries and chopped pistachios evenly over the batter.,Dollop and gently swirl remaining batter over the fruit, leaving some berries exposed.,Optional: sprinkle extra pistachios or coarse sugar on top.,Bake for 30–35 minutes, until the top is golden and a toothpick inserted into the center comes out with a few moist crumbs.,Allow bars to cool completely in the pan, then lift out using the parchment overhang and slice into squares.