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.