1. Preheat oven to 350°. Lightly grease bottom and sides of a 9-inch springform pan. Line bottom of pan with parchment paper.
2. Grate zest from lemons to equal 1 Tbsp. Cut lemons in half; squeeze juice from lemons into a bowl to equal 5 Tbsp. Reserve zest and 1 Tbsp. lemon juice.
3. Combine flour, sugar, and salt in bowl of a food processor; pulse 3 to 4 times or until blended. Add butter; pulse 6 to 7 times or until mixture resembles coarse crumbs. Reserve 1 cup flour mixture.
4. Transfer remaining flour mixture to bowl of a heavy-duty electric stand mixer. Add baking powder and baking soda; beat at low speed until well blended. Add buttermilk, egg, and 1/4 cup lemon juice; beat at medium speed 1 1/2 to 2 minutes or until batter is thoroughly blended, stopping to scrape bowl as needed. Stir in rosemary. Spoon half of batter into prepared pan.
5. Whisk lemon curd in a small bowl about 1 minute or until loosened and smooth; carefully spread over batter in pan. Top with remaining half of batter.
6. Stir together reserved lemon zest, 1 Tbsp. lemon juice, and 1 cup flour mixture; sprinkle lemon zest mixture over batter in pan.
7. Bake at 350° for 45 to 50 minutes or until a long wooden pick inserted in center comes out clean.
8. Let cool in pan on a wire rack 10 minutes. Gently run a sharp knife around edge of cake to loosen; remove sides of pan. Cool cake completely on wire rack (about 1 hour). Dust with powdered sugar just before serving.
*Greek yogurt may be substituted.
A 9-inch round cake pan may be substituted for springform pan. Line bottom and sides of cake pan with aluminum foil, allowing 2 to 3 inches to extend over sides; grease foil well. Proceed with recipe as directed through Step 7. Let cool in pan on a wire rack 10 minutes. Lift cake from pan, using foil sides as handles. Carefully remove foil. Cool and dust with powdered sugar as directed.