Preheat oven to 350°F. Grease an 8" round or square pan with butter. (A pie plate will also work nicely for this recipe.) Combine the flour, sugar, cinnamon, and baking soda in a mixing bowl. Stir to combine.
Add the butter to the dry ingredients and use a fork or pastry blender to mix until crumbly. (Little lumps are a good thing, don't worry about making it perfect.) Add the buttermilk to the bowl and stir again to combine. Spread the batter into the prepared pan.
Combine the butter, brown sugar, and cinnamon in a small bowl or large glass measuring cup. Microwave for 1 minute, or until the butter has melted. Stir to combine. Add the pecans and stir again. Sprinkle topping over the cake batter.
Bake for 35-38 minutes, until a toothpick inserted comes out clean or with moist crumbs. Serve warm or at room temperature.