Preheat oven to 350F. Lightly grease and flour a 9" x 5" loaf pan or three smaller loaf pans. Set aside. 

Make the filling by stirring dark brown sugar and ground cinnamon together in a small bowl. Set aside.  

To make the streusel topping, stir together the melted butter, flour, brown sugar, cinnamon and salt with a fork until combined. Set aside. 

In a large bowl, whisk together flour, sugar, baking soda and salt. 

In a medium bowl, whisk together egg, buttermilk and canola oil. Gradually add to dry ingredients and stir until combined. Pour half of the batter into the loaf pan (or distribute half of the batter between the three smaller loaf pans). Sprinkle filling evenly over the batter, then top with remaining batter and sprinkle with streusel topping.

Bake for 30 – 40 minutes for smaller loaves loafs and up to 60 for a large loaf. It will be done with a toothpick inserted into the middle comes out clean. Let cool on wire rack.