Preheat oven to 375 degrees.
Line a 9 inch square metal baking pan with parchment paper.
To make the crust: in a food processor combine flour, cornstarch, brown sugar, butter, baking powder, vanilla extract and salt, pulse until mixture resembles a fine meal.
Press the flour mixture into the bottom of the prepared pan. Bake at 375 for 15 minutes or until the edges of the crust are slightly browned.
While the crust makes, make the filling: Combine the syrup and the next 3 ingredients (through 1 tablespoon butter) in a medium saucepan; bring to a boil over medium heat. Cook 2 minutes, stirring constantly with a whisk until the sugar dissolves. Remove from the heat; add the pecans and 1/2 teaspoon vanilla, stirring with a whisk. Pour the mixture into the hot crust. Bake at 375 for 10 minutes or until the filling is bubbling. Cool completely in the pan. Cut into 20 bars.