Topping
1/4 cup all-purpose flour
1 1/2 cups firmly packed light brown sugar
2 tsps ground cinnamon
4 Tbsp unsalted butter, melted
1 cup finely chopped hazelnuts
In a bowl, whisk together the flour, brown sugar and cinnamon until blended. Stir in the butter until the topping is crumbly. Stir in the hazelnuts.
2 2/3 cups all-purpose flour
1 tsp baking powder
q tsp baking soda
1/2 tsp salt
1 cup unsalted butter, softened
1 cup, plus 2 Tbsp, granulated sugar
1 tsp vanilla
4 eggs
1 1/4 cups sour cream
In a bowl, whisk together the flour, baking powder, baking soda and salt until well blended; set aside.
In a stand mixer set on medium speed, beat the butter and sugar until creamy. Beat in the vanilla and the eggs, one at a time, until well blended, scraping the bowl as necessary. Add the flour mixture and sour cream alternately to the sugar mixture, begining and ending with the flour mixture. Scrape down the sides and bottom of bowl. The batter should be smooth and blended.
Spread the batter into the prepared baking pan and sprinkle the reserved topping over the batter. Bake until the cake has risen and browned and a toothpick inserted in the center comes out clean, 40 to 45 minutes. Let the cake cool in the pan for at least 15 minutes. Cut into 12 pieces and serve.