Preheat your oven to 350°F (175°C) and grease two 9x5-inch loaf pans.

Cream together the softened unsalted butter and granulated sugar until light and fluffy.

Add the eggs one at a time, then mix in the buttermilk until well combined.

In a separate bowl, whisk together the all-purpose flour and baking soda, then gently fold into the wet mixture.

Mix together brown sugar and ground cinnamon for the filling.

Layer the batter and cinnamon filling in the loaf pans.

Use a butter knife to swirl the filling into the batter.

Bake in the preheated oven for 45 to 50 minutes, checking for doneness.

Cool the loaves in the pans for 10 minutes before transferring to a wire rack.