Preheat oven to 350°F (180°C). Spray a 9-inch square baking pan with baking spray with flour; line with parchment paper, letting excess extend over sides of pan. Lightly spray parchment.

In the bowl of a stand mixer fitted with the paddle attachment, beat butter, granulated sugar, and lemon zest until fluffy, 3 to 4 minutes, stopping to scrape sides of bowl. Add eggs, one at a time, beating well after each addition. Beat in vanilla. (Mixture may look slightly broken at this point, but batter will come together.)

In a medium bowl, whisk together flour, poppy seeds, baking powder, salt, and baking soda. In a small bowl, whisk together milk and lemon juice. (Juice will cause milk to curdle slightly, and that is OK.) Add flour mixture to butter mixture alternately with milk mixture, beginning and ending with flour mixture, beating just until combined after each addition. Spoon batter into prepared pan, smoothing into an even layer. Tap pan on counter several times to spread batter and release any air bubbles.

Bake until a wooden pick inserted in center comes out clean, 30 to 40 minutes. Let cool in pan for 10 minutes. Using excess parchment as handles, remove from pan, and let cool completely on a wire rack.

Pour Crème Fraiche Glaze onto cake; using a small offset spatula, spread into an even layer. Garnish with poppy seeds and lemon zest, if desired.