Preheat oven to 350°F (175°C) and butter a 9-inch round cake pan. If using a springform pan, set it on a baking sheet.
Melt butter and stir in brown sugar until it forms a smooth syrup. Pour into the pan.
Let the syrup sit a minute, then arrange pecan halves over it in a single layer. Don't move them once placed.
Whisk flour, baking powder and salt. In a separate bowl, cream butter and sugar, beat in eggs and vanilla, then mix in the flour mixture and milk alternately.
Spoon batter gently over the pecans and bake 35 to 40 minutes until a toothpick comes out clean.
Cool 5 to 10 minutes while the caramel is still warm and glossy, then run a knife around the edge and flip onto a plate.