1. Preheat oven to 325 degrees. Butter a 9-inch round cake pan. Line bottom with parchment cut to fit, and butter parchment. Dust with flour, and tap out excess.

2. Combine sour cream, vanilla, zest, and juice. In a separate bowl, whisk flour, baking powder, baking soda, and salt.

3. Beat butter and sugar with a mixer on medium-high speed until pale and fluffy. Add eggs and whites, 1 at a time, beating well after each addition. Beat in sour cream mixture. Reduce speed to low, and beat in flour mixture until just combined. Transfer to pan, and smooth top.

4. Bake until a toothpick comes out clean, 40 to 45 minutes. Let cool in pan on a wire rack for 5 minutes, then turn out cake onto rack to cool completely. (Cake can be stored in an airtight container at room temperature overnight.) Before serving, dust with confectioners' sugar, and arrange flowers on top.