Preheat the oven to 325. Generously butter a 9x13 baking dish. (For thicker bars, you can use an 11x7 or a 9x9 inch pan.)
Stir together the flour, sugar, baking powder, and salt. Stir in melted butter.
Press the cake mix into the bottom of the baking dish.
Beat the eggs into the cream cheese, one at a time, until smooth. Beat in vanilla and powdered sugar.
Spread filling over cake mix. Bake 40-50 minutes, until the edges are brown and lightly crisped.
Remove from oven and dust with powdered sugar before serving.