Servings: Serves 10 (large batch: 30)
Ingredients 3/4 cup (1 1/2 sticks) butter , at room temperature, plus more for greasing (large batch: 2 1/4 cups or 4 1/2 sticks)1 1/2 cups all-purpose flour , plus more for dusting (large batch: 4 1/2 cups)1/2 cup light brown sugar (large batch: 1 1/2 cups)4 tsp. freshly grated ginger (large batch: 12 tsp.)4 tsp. vanilla extract , divided (large batch: 12 tsp.)3/4 tsp. fine salt , divided (large batch: 2 1/4 tsp.)2 to 2 1/2 cups mixed green, red and black seedless grapes (large batch: 6 to 7 1/2 cups)2 tsp. baking powder (large batch: 6 tsp.)1/2 cup granulated sugar (large batch: 1 1/2 cups)2 eggs (large batch: 6 eggs)1/2 cup buttermilk (large batch: 1 1/2 cups)
Active time: 30 minutes
Total time: 1 hour, 40 minutes
Preheat oven to 350°. Grease a 9-inch round cake pan with butter and dust with flour, shaking off any excess; set aside. In a medium bowl, stir together 1/4 cup butter, brown sugar, ginger, 2 teaspoons vanilla, and 1/4 teaspoon salt. Spread over the bottom of the pan and then top with grapes, arranging them in a snug, single layer; set aside.
In a medium bowl, whisk together flour, baking powder, and remaining 1/2 teaspoon salt; set aside. In a large bowl, beat remaining 1/2 cup butter and granulated sugar with an electric mixer until pale and fluffy, about 1 minute. Add eggs, one at a time, then remaining 2 teaspoon vanilla. Add flour mixture in thirds, alternating with buttermilk, and beating until smooth. Spoon batter over grapes, spread out evenly, and bake until golden brown and the middle of the cake springs back when touched, 40 to 50 minutes.
Set aside to let cool for 20 minutes, then loosen around the edges and carefully invert onto a plate.