Preheat oven to 350°F (175°C). Grease a 9×13-inch pan.
In a bowl, whisk flour, baking powder, baking soda, salt, and poppy seeds.
In a separate bowl, cream butter and sugar until fluffy.
Beat in eggs, one at a time, followed by lemon zest and juice.
Mix in sour cream and vanilla.
Gradually fold in dry ingredients until just combined.
Spread batter evenly in the prepared pan.
Bake 30–35 minutes until golden and a toothpick inserted comes out clean.
Cool completely before glazing or frosting.
