Line 12 muffin tins and set aside.
Add flour, yeast, sugar, salt and cinnamon into a mixer bowl. MIx on low speed until well combined. Add buttermilk, melted butter and egg and mix on medium speed until well combined. Fold in chocolate chunks and walnuts. Spoon into lined muffin tin.
Mix together all of the streusel topping ingredients until crumbled. Sprinkle over muffin batter.
Cover and allow to rise 45 minutes to 1 hour.
Preheat oven to 375 degrees.
Bake for 25-30 minutes or until a toothpick inserted comes out mostly clean.
Remove to a wire rack to cool.
Best served at room temperature.