Preheat your oven to 350°F (175°C) and prepare a 13x18 inch sheet pan by greasing and flouring it.

In a large bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a separate large bowl, cream together the softened butter and granulated sugar until light and fluffy, about 3-5 minutes.

Beat in the eggs one at a time, ensuring that each egg is fully incorporated before adding the next. Scrape down the sides of the bowl as needed.

Add the sour cream, lemon juice, lemon zest, and vanilla extract. Beat until just combined.

Gradually add the dry ingredients to the wet ingredients, alternating with the buttermilk. Start and end with the dry ingredients, mixing until just combined after each addition. Be careful not to overmix.

Pour the batter into the prepared sheet pan, spreading it evenly.

Bake for 20-25 minutes, or until a wooden skewer inserted into the center comes out clean.

Once baked, remove the pan from the oven and let the cake cool completely in the pan on a wire rack.

While the cake cools, prepare the glaze by whisking together the powdered sugar, lemon juice, melted butter, and lemon zest (if using) in a medium bowl until smooth. Adjust the lemon juice to achieve your desired consistency.

Once the cake is completely cool, pour the lemon glaze over the top and use a spatula to spread it evenly.

Allow the glaze to set for at least 30 minutes before cutting and serving.