- 1 small fennel bulb
- 3/4 cup sugar
- 3/4 cup water
- 3 (3-inch) strips lemon zest, thinly sliced
- 1 teaspoon fennel seeds
- 1 1/2 cups all-purpose flour
- 1 1/2 teaspoons baking powder
- 1 teaspoon baking soda
- 1/2 teaspoon salt
- 1 stick unsalted butter, softened
- 3/4 cup sugar
- 2 large eggs
- 1 1/2 teaspoons grated lemon zest
- 3/4 cup well-shaken buttermilk
- Equipment: a 9-by 2-inch round cake pan; an adjustable-blade slicer
- Accompaniment: lightly sweetened whipped cream
- Lightly oil pan and line bottom and side with a large round of wax paper, pleating sides and trimming to fit. Lightly oil paper. Line side with a 2-inch-wide strip of wax paper long enough to wrap around inside of pan to cover pleats, then lightly oil.
- Cut fennel bulb lengthwise with slicer into enough 1/4-inch-thick slices (about 9) to cover bottom of cake pan.
- Cover fennel with cold water in a medium saucepan and bring to a boil. Drain fennel and set aside. Add sugar, water (3/4 cup), zest, and fennel seeds to saucepan and bring to a simmer, stirring until sugar has dissolved. Add fennel slices and very gently simmer until tender and translucent and liquid is syrupy, about 40 minutes. Lift fennel slices out with a fork and arrange decoratively in bottom of cake pan. If you have more than 1/3 cup syrup, boil to reduce; if less, add water. Cool syrup slightly, then pour (through a fine-mesh sieve if desired) over fennel.
- Preheat oven to 350°F with rack in middle.
- Whisk together flour, baking powder, baking soda, and salt. Beat together butter and sugar in a large bowl with an electric mixer at high speed until light and fluffy. Add eggs 1 at a time, beating well after each addition, then beat in zest.
- At low speed, mix in flour mixture in 3 batches, alternating with buttermilk, beginning and ending with flour mixture, and mixing until just combined. Gently spoon batter over topping, spreading evenly.
- Bake until cake is golden-brown and a wooden pick inserted into center of cake comes out clean, 25 to 30 minutes. Cool cake in pan 15 minutes, then invert onto a plate and continue to cool. Serve warm or at room temperature.