Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large bowl, whisk together the flour, granulated sugar, baking powder, and salt.
In another bowl, cream together the softened butter, egg, and vanilla extract until smooth.
Gradually add the dry ingredients to the wet mixture, alternating with the milk, mixing until just combined.
Gently fold in the blueberries and lemon zest, if using.
In a separate bowl, prepare the streusel topping by combining the flour, brown sugar, melted butter, and cinnamon until crumbly.
Pour the batter into the prepared baking pan and sprinkle the streusel topping evenly over the batter.
Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted in the center comes out clean.
Allow the coffee cake to cool in the pan for about 10 minutes before slicing and serving.