Instructions

To make the cake:

1. Preheat oven to 350°F. Grease a 9"x 13"x 2" baking pan. If you like, line the pan with parchment paper and grease the paper and pan.
2. Place the hazelnuts and 1/4 cup (30g) flour in the bowl of a food processor. Pulse until finely ground.
3. Whisk together the remaining 2 & 1/4 cups (270g) flour, baking powder, baking soda, salt, and hazelnut mixture. Set aside.
4. Using an electric mixer on medium speed, beat the butter, sugar, and brown sugar until light and fluffy. Add the chocolate-hazelnut spread and vanilla. Continue mixing until combined. Add the eggs, one at a time, mixing well after each addition.
5. Reduce mixer speed to low. Add the flour mixture in 3 portions, alternating with 2 portions of sour cream. Mix just until combined.
6. Transfer the batter to the prepared pan and spread evenly.
7. Bake 25 to 30 minutes, or until a pick inserted into the center comes out clean. Cool the cake completely in the pan.

To make the frosting:

1. Using an electric mixer on medium speed, beat the butter, cream cheese, and chocolate-hazelnut spread until smooth.
2. Gradually add the confectioners' sugar, beating until smooth.
3. Spread the frosting on top of the cooled cake. (You can first remove the cake from the pan if you prefer.) Garnish with coarsely chopped toasted hazelnuts.