Preheat oven to 350°F (175°C). In a 9-inch round cake pan, pour melted butter, sprinkle brown sugar evenly, then arrange pecan halves in a single layer.

In a medium bowl, whisk flour, baking powder, salt, and cinnamon together until free of lumps.

Beat room-temperature butter with granulated sugar until light and fluffy, about 2–3 minutes.

Add eggs one at a time, mixing well after each, then stir in vanilla.

Alternate adding flour mixture and milk to butter mixture, beginning and ending with flour; mix until just combined.

Spoon batter over pecans, smoothing top gently, and bake for 30–35 minutes or until a toothpick comes out with moist crumbs.

Let cake rest in pan for 10 minutes, run a knife around edges, then invert onto serving plate.

Slice warm or at room temperature; top with vanilla ice cream if desired.