Winemakers Grape Cake

Name: Winemakers' Grape Cake
Ingredients: 4 tablespoons unsalted butter, melted, plus more butter for pan; 1 1/2 cups unbleached all-purpose flour, plus more for pan; 2 large eggs, room temperature; 2/3 cup granulated sugar; 1/4 cup extra-virgin olive oil; 1/3 cup whole milk; 1/2 teaspoon pure vanilla extract; 3/4 teaspoon baking powder; 1 pinch of sea salt; Grated zest of 1 lemon; Grated zest of 1 orange; 10 ounces small fresh purple grapes; Confectioners' sugar, for garnish