Step 1
Preheat oven to 350°F. Grease and flour one 10-inch tube pan.
Step 2
In a small bowl, whisk flour, baking powder, baking soda, and salt.
Step 3
In a large bowl, beat sugar and butter until light and fluffy. About 4 minutes. Add eggs, one at a time. Add vanilla. Scrape the sides of the bowl as needed. Add flour mixture and sour cream alternately into the mixture. Beginning and ending with flour mixture. Add grapes and hazelnuts to the batter and incorporate using a spatula. Pour mixture into the prepared pan.
Step 4
In a medium bowl, mix together oats, flour, sugar, cinnamon, and salt. Using your fingers, cut the butter into the mixture. Add hazelnuts. Sprinkle the topping over the cake.
Step 5
Bake for 35-40 minutes or until a toothpick inserted in the middle comes out clean. Remove from oven and cool for 10 minutes. Remove from pan and place cake on a serving platter.
Step 6
While the cake is cooling down, whisk the powdered sugar and the milk together until smooth. Adjust the amount of milk to achieve a thick glaze as needed. Using a spoon, drizzle over the cake. Serve.
