Prepare the Topping:
Preheat the oven to 350°F (175°C). In a cast-iron skillet or oven-safe pan, melt butter over medium heat. Stir in brown sugar until smooth. Spread pecans evenly over the caramel mixture.
Make the Cake Batter:
In a bowl, cream softened butter and sugar until light and fluffy. Beat in eggs one at a time, then add vanilla extract.
In a separate bowl, whisk together flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, alternating with milk. Mix until just combined.
Assemble and Bake:
Pour the cake batter over the pecan topping, ensuring even coverage. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Flip and Serve:
Let the cake cool in the pan for 10 minutes. Carefully invert onto a serving plate. Serve warm or at room temperature.