Coat a 9-inch round cake pan with cooking spray. Place 6 or 7 geranium leaves, dull side up, in bottom of pan; set aside.
Mince remaining 2 or 3 leaves, and set them aside.
Beat butter and sugar at medium speed with an electric mixer until fluffy. Add egg yolks, 1 at a time, beating until blended after each addition. Stir in lemon juice. Add milk, and beat at low speed until blended. (Batter will look curdled.)
Combine flour and baking powder. Gradually add flour mixture to butter mixture, beating at low speed until blended. Stir in lemon zest and minced leaves.
Beat egg whites until soft peaks form; gently fold into batter. Pour batter into prepared pan.
Bake at 350° for 1 hour and 20 minutes or until a wooden pick inserted in center comes out clean. Cool in pan on wire rack 30 minutes. Invert to remove cake; cool completely on wire rack. Dust with powdered sugar.