A rustic polenta cake is studded with red grapes and rosemary.
Preheat oven to 350 degrees. Brush bottom and sides of an 8-inch square baking pan with butter. Line bottom and two sides with parchment, leaving a 2-inch overhang; butter parchment.
Melt butter in a small saucepan over medium. Simmer, swirling pan occasionally, until butter turns golden brown and nutty-smelling, 8 to 10 minutes. Transfer to a small bowl; let cool completely. Meanwhile, whisk together flour, polenta, baking powder, and salt in a medium bowl.
Beat cooled butter, granulated sugar, and honey on medium-high speed until mixture resembles a sandy paste, 2 to 3 minutes. Beat in eggs one at a time, scraping down sides of bowl as necessary. Reduce speed to low and beat in flour mixture in two batches, alternating with milk, just until combined (do not overmix). Fold in 1 cup grapes.
Pour batter into prepared pan, smoothing top. Bake 15 minutes. Scatter remaining 1 cup grapes, rosemary, and sanding sugar over top. Bake until golden and a tester inserted in center comes out clean, 30 to 35 minutes. Let cool in pan on a wire rack 15 minutes; remove cake using parchment and let cool completely on rack. Serve warm or room temperature.