Haven't made this yet but it looks good
https://www.bonappetit.com/recipe/coffee-cake-chocolate-streusel
Ingredients Chocolate Streusel
1 cup chopped walnuts
1 cup semisweet chocolate chips
½ cup (packed) light brown sugar
2 teaspoons ground cinnamon
Pinch of kosher salt
Cake and Assembly
1 cup (2 sticks) unsalted butter, room temperature, plus more for pan
4 cups all-purpose flour, plus more for pan
2 teaspoons baking powder
2 teaspoons baking soda
½ teaspoon kosher salt
2 cups granulated sugar
4 large eggs
2 cups sour cream
2 teaspoons vanilla extract
Special Equipment
A 12-cup Bundt pan
Recipe Preparation Chocolate Streusel
Combine walnuts, chocolate chips, brown sugar, cinnamon, and salt in a large bowl.
Cake and Assembly
Preheat oven to 350°. Butter and flour Bundt pan, tapping out excess flour. Whisk 4 cups flour, baking powder, baking soda, and salt in a medium bowl. Using an electric mixer on medium-high speed, beat granulated sugar and 1 cup butter in a large bowl until light and fluffy, about 5 minutes. Add eggs one at a time, beating to blend and scraping down sides of bowl after each addition. Reduce speed to low and add dry ingredients in 2 additions, alternating with sour cream. Add vanilla and beat just to combine.
Transfer half of batter to prepared pan; smooth surface. Sprinkle half of streusel evenly over batter. Scrape in remaining batter, smooth surface, and sprinkle with remaining streusel. Bake cake until golden brown and a tester inserted into the center comes out clean, 70–80 minutes. Transfer pan to a wire rack and let cake cool in pan 30 minutes before inverting onto wire rack. Let cool completely.
Do Ahead: Cake can be made 3 days ahead. Store tightly covered at room temperature.