Preheat the oven to 350°. Butter a 9-inch cake pan and line it with a 9-inch round of parchment paper. Butter the parchment paper.
In a bowl, sift together the cake flour, baking powder and salt. In a large bowl, using a handheld mixer, beat the butter with both sugars at medium-high speed until light and fluffy. Add the yolks, one at a time, beating well between additions. Beat in the lemon zest and vanilla extract. At low speed, alternately beat in the flour mixture and the buttermilk until almost blended. With a rubber spatula, finish folding the mixture together just until smooth. Scrape the batter into the prepared pan and bake for about 35 minutes, until a cake tester inserted in the center comes out clean. Transfer the cake to a rack to cool completely.
Peel and halve the pears. With a spoon, scoop out the cores and discard. In a large saucepan, combine the Riesling, water, sugar, vanilla bean and seeds, orange zest and star anise pods. Bring to a boil, stirring to dissolve the sugar. Add the pear halves and simmer over moderate heat, turning once, until tender, about 5 minutes. Transfer the pears to a large plate. Discard the star anise, vanilla bean and orange zest. Boil the Riesling poaching liquid over high heat until reduced to 1 cup and syrupy, about 15 minutes. When the pears have cooled, slice them lengthwise 1/3 inch thick.
In a small bowl, combine the crème fraîche, vanilla extract, vanilla seeds and confectioners' sugar.
Unmold the cake onto a plate and peel off the paper. Cut the cake into 10 wedges and transfer to plates. Place a sliced pear half alongside each cake slice. Spoon some of the Riesling syrup over the cake, top with some vanilla crème fraîche and serve.