Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or baking spray, then line the bottom with parchment paper for easy removal.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until evenly combined. Set aside.

In a large mixing bowl, beat the softened butter and sugar together with an electric mixer on medium-high speed for 3-4 minutes until light and fluffy. The mixture should look pale and increased in volume.

Add the eggs one at a time, beating well after each addition. Scrape down the sides of the bowl, then mix in the lemon zest, lemon juice, and vanilla extract until fully incorporated.

Reduce the mixer speed to low and add half of the flour mixture, mixing just until the streaks disappear. Add the sour cream and mix until combined, then add the remaining flour mixture and mix just until no dry pockets remain. Do not overmix.

Pour the batter into the prepared pan and smooth the top with an offset spatula. Bake for 30-35 minutes, or until the top is golden and a toothpick inserted in the center comes out with just a few moist crumbs.

While the cake is still warm in the pan, use a wooden skewer to poke 20-30 holes evenly across the surface, going about halfway down into the cake.

Make the glaze by whisking together the powdered sugar and 3 tablespoons of lemon juice in a medium bowl until smooth and pourable. Add the remaining tablespoon of juice if the glaze is too thick—it should drizzle off a spoon in a thick ribbon.

Immediately pour the glaze evenly over the warm cake, using the back of a spoon to gently spread it to the edges. The glaze will pool in the holes and soak into the cake as it cools.

Let the cake cool completely in the pan, uncovered, for at least 2 hours to allow the glaze to set into a glossy finish. Slice into squares and serve at room temperature.