If you like, serve this with whipped cream or an extra dollop of yogurt.
Ingredients
1/2 cup (1 stick) unsalted butter, room temperature, plus more for pan
1/2 cup blanched hazelnuts, chopped
2 ounces bittersweet chocolate, chopped
3/4 teaspoons ground cinnamon
1 cup sugar, divided
2 cups all-purpose flour
3/4 teaspoons baking powder
1/2 teaspoons baking soda
1/4 teaspoons kosher salt
1 teaspoon vanilla extract
2 large eggs
1 cup plain whole Greek yogurt
Directions
Preheat oven to 350°F. Butter 8" square baking pan; set aside. Toss hazelnuts, chocolate, cinnamon, and 1/4 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 3/4 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.
DO AHEAD: Cake can be made 1 day ahead. Store airtight at room temperature.
