½ cup (1 stick) Unsalted Butter, room temperature, plus more for pan
½ cup blanched Hazlenuts, chopped
2 oz. bittersweet Chocolate, chopped
¾ tsp. Ground Cinnamon
1 cup Sugar, divided
2 cups All-Purpose Flour
¾ tsp. Baking Powder
½ tsp. Baking Soda
¼ tsp. Kosher Salt
1 tsp. Vanilla Extract
2 large Eggs
1 cup plain whole Greek Yogurt
Instructions
Preheat oven to 350°. Butter 8” square baking pan; set aside.
Toss hazelnuts, chocolate, cinnamon, and ¼ cup sugar in a bowl.
Whisk flour, baking powder, baking soda, and salt in a medium bowl.
Using an electric mixer on high speed, beat butter and remaining ¾ cup sugar in a large bowl until light and fluffy, about 3 minutes.
Beat in vanilla. Add eggs one at a time, beating to blend between additions and occasionally scraping down sides of bowl.
With mixer on low speed, add dry ingredients in 3 additions, alternating with yogurt in 2 additions, beginning and ending with dry ingredients.
Pour half of batter into prepared pan and smooth top; top with half of hazelnut mixture. Spoon remaining batter over and smooth top; top with remaining hazelnut mixture.
Bake cake until top is golden brown and a tester inserted into the center comes out clean, 40–45 minutes.
Transfer pan to a wire rack; let cake cool in pan before turning out.