Adapted from Bake from Scratch
Preheat oven to 350F. Spray a 9-inch square baking pan with baking spray. Line with parchment paper, letting excess extend over sides of pan; lightly spray the parchment.
In a medium bowl, whisk together flour, poppy seeds, baking powder, salt, and baking soda. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter, sugar, and lemon zest at medium speed until fluffy, 3 to 4 minutes, stopping to scrape sides and bottom of bowl as needed.
Add the eggs, one at a time, beating well after each addition and scraping the sides and bottom of the bowl. Beat in the vanilla. (Mixture may look slightly broken at this point, but batter will come together.)
In a small bowl, whisk together milk and lemon juice. (Don't worry about curdling.) With mixer on low speed, gradually add flour mixture to the butter mixture alternately with milk mixture, beginning and ending with flour mixture, mixing just until combined after each addition.
Scrape batter into prepared pan, smoothing into an even layer. Tap pan once or twice to settle batter and release any air bubbles.
Bake until a cake tester inserted in the center comes out clean, 30 to 40 minutes. Let cool in the pan for 10 minutes. Use the parchment sling to remove the cake from the pan, and let cool completely on a wire rack.
Whisk together the glaze ingredients except the poppy seeds. Taste--I like my glazes quite tart, so I added a little extra sour cream and lemon juice.
When the cake is cool, place wax paper or foil under the cooling rack. Pour the glaze over the cake, reaching all of the surface and letting it drip over the sides. If you have a lot extra pooling underneath the cake, use the wax paper or foil to gather and pour again.
Sprinkle with poppy seeds. Enjoy.