Grease 12 cupcake liners, place in muffin tin. Set aside.
Add flour, yeast, sugar, salt and cinnamon into a mixer bowl fitted with a paddle. Mix on low speed until well combined. Add buttermilk, melted butter and egg; mix on medium speed until well combined. Fold in chocolate chunks and walnuts. Spoon evenly into lined muffin tin.
Prepare topping: 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°F.
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 (if you can wait until it cools!)