For the chocolate pecan filling and topping: Place brown sugar, pecans and cinnamon in food processor. Cover and pulse until nuts are coarsely chopped. Transfer 3/4 cup of mixture to small bowl and stir in chopped chocolate for filling, set aside. To remaining mixture in food processor add flour, cocoa powder and butter and pulse until mixture is crumbly for topping.

For the cake: Place a rack in the center of the oven and preheat oven to 325°F. Butter bottom and sides of 9-inch springform pan or a 9x9x2 square pan. Spray pan with cooking spray and set aside.Sift flour, baking powder, baking soda and salt into medium bowl and set aside. In an electric mixer fitted with the paddle attachment, beat butter at medium-high speed in mixing bowl for 1 minute or until creamy. Gradually add sugar and beat at high speed until well blended and light, about 2 minutes. Add eggs one at a time, beating well after each addition and scraping down sides of bowl as necessary. Beat in vanilla extract. Reduce speed to low and add 1/3 of the flour mixture. When incorporated add the buttermilk, then another 1/3 of the flour mixture, followed by the greek yogurt. And end with the flour mixture. Mix until just combined. Do not overmix.

Bake: Scrape half of batter into prepared pan and smooth top. Sprinkle with filling. Scrape remaining batter over filling and smooth top. Sprinkle with topping. Bake 50 to 60 minutes or until toothpick inserted into center of cake comes out clean or with just a few crumbs. Mine took 55 minutes. Let cake cool in pan set on wire rack for 10 minutes. Remove side of pan and cool completely before serving (about 2 hours).