Preheat your oven to 375 degrees and grease and line a 9x13 baking pan with parchment paper. If you plan on serving the cake within the pan, omit the parchment paper. In a medium bow, sift the flour, baking powder, salt, and set aside.
Using an electric mixer with the paddle attachment, beat the butter until creamy. Gradually add the sugar and beat until light and fluffy.
Add the egg yolks, one at a time and beat until combined.
Scrape down the sides of the bowl and add vanilla, lemon juice, lemon zest and poppy seeds. Beat until combined.
With the mixer one low, add the flour and milk alternately until beat until combined.
Pour the batter into prepared pan and using a spatula, spread until even in the pan. The batter will be a little sticky and will need to be pushed lightly into the corners of the pan.
Bake for 25 - 30 minutes or until a toothpick in the center comes out clean.
Let the cake cool completely before frosting. If you would like to remove the cake from the pan, allow to cool for at least 30 minutes before doing so.