Preheat your oven to 375°F (190°C). Line a 12-cup muffin tin with paper liners or lightly grease each cup to prevent sticking.
In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and 1 teaspoon ground cinnamon.
In a separate bowl, beat the softened unsalted butter until smooth. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Alternately add the dry ingredients and buttermilk to the butter mixture, starting and ending with the dry ingredients. Mix just until combined.
In a small bowl, mix the packed brown sugar with 1 tablespoon ground cinnamon to create the cinnamon sugar swirl.
Spoon about a tablespoon of muffin batter into each muffin cup. Sprinkle a teaspoon of the cinnamon sugar mixture on top, then add another tablespoon of batter. If using, sprinkle chopped turkey bacon and walnuts on top, then finish with a final teaspoon of cinnamon sugar.
Use a toothpick or skewer to gently swirl the cinnamon sugar into the batter, creating cinnamon ribbons.
Bake for 18-22 minutes or until a toothpick inserted into the center comes out clean. Muffins should be golden brown on top.
Allow the muffins to cool in the tin for about 5 minutes before transferring to a wire rack. Serve warm or at room temperature.