Make the streusel:

1. In a medium bowl, mix together chopped walnuts, chocolate chips, brown sugar, cinnamon, cocoa powder and salt. Set aside.

Make the Cake:

1. Preheat the oven to 350F. Butter and flour a 12-inch bundt cake, tapping out excess flour.
2. In a medium bowl, whisk flour, baking powder, baking soda, and salt in a medium bowl.
3. In the bowl of a stand mixer fitted with the paddle attachment, cream butter and sugar on medium-high speed, until light and fluffy.
4. Add eggs one at a time, scraping down sides of bowl after each addition.
5. Reduce speed to low and add dry ingredients in 2 additions, alternating with the greek yogurt. Add vanilla and beat just to combine.
6. Pour half the batter onto the prepared bundt pan. Sprinkle half of the chocolate streusel mix over the batter.
7. Transfer the rest of the batter over, and sprinkle 1/4 of the remaining streusel.
8. Bake for 70 minutes, or until a toothpick inserted in the center comes out mostly clean (melted chocolate is ok).
9. Let cool before transferring to a cooling rack.
10. Once cooled, sprinkle the remaining chocolate streusel. Dust some powdered sugar, if desired.