- Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking dish or line it with parchment paper. 
- In a large mixing bowl, whisk together the flour, sugar, baking powder, and salt until well combined. 
- In another bowl, mix the melted butter, milk, eggs, and vanilla extract until smooth. 
- Pour the wet ingredients into the dry ingredients and stir until just combined. Be careful not to overmix; a few lumps are okay. 
- Gently fold in the blueberries and lemon zest, if using. 
- Pour the batter into the prepared baking dish and spread it evenly. 
- Bake for 30-35 minutes or until a toothpick inserted into the center comes out clean. 
- Allow the cake to cool in the pan for about 10 minutes before slicing and serving.