MAKE THE CAKE: Preheat the oven to 325 degrees. Butter and flour a 9-by-5-inch metal loaf pan. In a medium bowl, whisk the 1 3/4 cups of cake flour with the polenta and salt. In a large bowl, using an electric mixer, beat the softened butter with the sugar at medium-high speed until pale and fluffy, about 2 minutes. Add the eggs one at a time, beating well between additions. Beat in the vanilla extract and lemon juice. Gradually add the flour mixture to the egg mixture, beating the batter at low speed until just incorporated.

Scrape the batter into the prepared loaf pan and bake for about 1 hour, until a cake tester inserted in the center of the cake comes out with a few moist crumbs attached. Turn the pound cake out onto a rack and let it cool until warm.

MEANWHILE, MAKE THE ROSEMARY SYRUP: In a small saucepan, combine the honey with the water and rosemary sprigs and simmer over moderate heat for 2 minutes. Let the syrup cool to room temperature, about 15 minutes. Discard the rosemary sprigs. While the cake is still warm, lightly brush the top of the cake with the rosemary syrup.

MAKE THE MASCARPONE: In a medium bowl, whisk the mascarpone with the cream, sugar, and vanilla-bean seeds.

Using a serrated knife, cut the pound cake into thick slices and transfer to plates. Top each slice with a large dollop of mascarpone, drizzle with the rosemary syrup, and serve with sliced figs, if using.

The polenta pound cake can be kept, covered, at room temperature for up to 2 days. The rosemary syrup and mascarpone can be refrigerated separately for up to 2 days.