Begin by preheating your oven to 350°F (175°C). Generously grease a 9-inch round cake pan with butter and set it aside to prepare for the filling.
In a small saucepan over medium heat, melt the 1/2 cup of unsalted butter. Once melted, add the packed brown sugar, stirring continuously until the mixture is fully dissolved and starts to bubble. Remove from heat and pour the mixture into the prepared cake pan, ensuring that it spreads evenly across the bottom.
After the brown sugar layer is in place, evenly sprinkle the roughly chopped pecans over the buttery mixture.
In a separate mixing bowl, whisk together the all-purpose flour, baking powder, salt, and ground cinnamon (if using). Set this flour mixture aside for later use.
In another large mixing bowl, cream together the granulated sugar and remaining 1/2 cup of softened butter. Beat them together until the mixture is light and fluffy.
Add the eggs to the creamed mixture one at a time, ensuring to mix well after each addition. Then, incorporate the vanilla extract, mixing until fully blended.
Gradually add the reserved flour mixture to the butter and sugar mixture, alternating with the buttermilk. Start and finish with the flour mixture, mixing only until just combined to maintain a tender cake texture.
Carefully pour the prepared cake batter over the layered pecans and brown sugar in the cake pan, making sure to spread the batter evenly across the surface.
Bake the cake in your preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean, indicating it is fully cooked.
Once baked, remove the cake from the oven and allow it to cool in the pan for about 10 minutes for easier removal.
To invert the cake, carefully run a knife around the edges of the pan. Then, place a serving plate or platter on top of the cake pan and swiftly flip it to let the cake fall out, allowing the delicious pecan and sugar mixture to adorn the top.
Notes
Serve warm with whipped cream or vanilla ice cream for an elegant touch.