Preheat the oven to 350F.
In a medium bowl, beat the butter and sugar together. Add the flour, baking powder and salt and blend until crumbly; press into the bottom of a parchment-lined 8x8-inch pan. Bake for 10-12 minutes, or until very pale golden around the edges.
In the same bowl, stir together the brown sugar, butter, birch or maple syrup and pecans. Pour over the base and bake for 25-30 minutes, until bubbly on top. Cool completely in the pan on a wire rack before cutting into squares.
Makes 16 squares.