Follow these straightforward steps to create your delightful Lemon Blueberry Sheet Cake:

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 9×13-inch baking pan.
2. Mix Dry Ingredients: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
3. Cream Butter and Sugar: In a large bowl, cream the softened butter and granulated sugar until light and fluffy.
4. Add Eggs: Beat in the eggs, one at a time, ensuring each is fully incorporated before adding the next.
5. Combine Wet Ingredients: Mix in the milk, lemon zest, lemon juice, and vanilla extract until combined.
6. Combine Mixtures: Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to overmix.
7. Fold in Blueberries: Gently fold in the blueberries, ensuring they are evenly distributed throughout the batter.
8. Pour Batter: Pour the batter into the prepared baking pan, smoothing the top with a spatula.
9. Bake: Place the pan in the preheated oven and bake for 35-40 minutes or until a toothpick inserted in the center comes out clean.
10. Cool: Once baked, remove the cake from the oven and let it cool in the pan for 15 minutes. Then, transfer it to a wire rack to cool completely.
11. Prepare for Serving: Once cooled, dust with powdered sugar, and garnish with lemon slices and additional blueberries if desired.

These steps will guide you through making an amazing Lemon Blueberry Sheet Cake that is sure to impress!