Preheat your oven to 350°F (175°C). Grease your 9x9-inch baking pan with butter or non-stick spray and lightly dust with flour to prevent sticking.
In a medium bowl, whisk together the all-purpose flour, granulated sugar, baking powder, ground cinnamon, and salt. This ensures the leavening and spices are evenly distributed.
Using an electric mixer or a sturdy whisk, cream the softened butter until smooth and fluffy. Add the eggs one at a time, beating well after each addition. Then, mix in the vanilla extract.
Gradually add the dry ingredients to the butter mixture in three parts, alternating with the milk in two parts. Start and end with the dry ingredients. Mix gently after each addition until just combined — overmixing can make the cake tough.
In a small bowl, stir together the brown sugar, chopped walnuts or pecans, and a sprinkle of cinnamon if you desire a more intense flavor. This mixture will add a sweet, crunchy topping.
Pour half of the batter into your prepared pan, spreading it evenly. Sprinkle half of the cinnamon nut topping over the batter. Then add the remaining batter on top, smoothing it out gently. Finish by sprinkling the remaining topping across the surface.
Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean. The cake should be golden brown on top and smell wonderfully fragrant.
Allow the cake to cool in the pan for about 15 minutes, then transfer to a cooling rack. Serve warm or at room temperature, ideally with your favorite cup of coffee or tea.