Preheat oven to 350°F.
In a medium bowl, soft together the flour, baking powder and a pinch of salt. Grate the lemon zest over the flour mixture. Set aside.
In the bowl of an electric mixer fitted with the whisk attachment, Beat together the butter and sugar until they lighten in color, 5-8 minutes. Slowly beat in the eggs, olive oil and milk. Remove the bowl from the mixer and use a rubber spatula to fold the flour into the eggs. Fold in about 1 cup of the grapes. Stir only enough to blend.
Thoroughly grease the bottom and sides of the cake pan with the extra butter. Pour the batter into the pan and place it in the center of the oven. Bake for 40-45 minutes, until a toothpick inserted in the center comes out clean. Meanwhile, combine the honey and ginger in a small sauté pan. Bring to a light simmer over low heat. When it bubbles, thickens and turns light brown, 2-3 minutes, add the remaining grapes. Remove from the heat and allow to cool. Serve slices of the cake with some of the grape-ginger mix spooned over it and a dollop of sour cream.