For streusel: In a medium bowl, whisk together flour, sugar, and poppy seeds. Using a pastry cutter or 2 forks, cut in cold butter until incorporated and mixture is crumbly. Refrigerate until ready to use, at least 30 minutes.

Preheat oven to 375°F (190°C). Spray an 8½x4½- inch loaf pan with baking spray with flour. Line pan with parchment paper, letting excess extend over all sides of pan.

For batter: In the bowl of a stand mixer, whisk together sugar and lemon zest by hand until sugar is fragrant. Add butter; using the paddle attachment, beat at medium speed until light and fluffy, 3 to 4 minutes, stopping to scrape bottom and sides of bowl. Add eggs, one at a time, beating well after each addition and stopping to scrape sides of bowl. Beat in vanilla.

In a medium bowl, whisk together flour, poppy seeds, baking powder, and salt. In a small bowl, whisk together buttermilk and lemon juice. With mixer on low speed, gradually add flour mixture to sugar mixture alternately with buttermilk mixture, beginning and ending with flour mixture, beating just until combined after each addition. Spread batter into prepared pan. Firmly tap pan on a kitchen towel-lined counter to settle batter. Top batter with streusel.

Bake for 20 minutes. Reduce oven temperature to 350°F (180°C), and bake until a wooden pick inserted in center comes out with a few moist crumbs and an instant-read thermometer inserted in center registers at least 205°F (96°C), 50 to 55 minutes more. Let cool in pan for 15 minutes. Remove from pan, and let cool completely on a wire rack. Store in an airtight container for up to 3 days.