Preheat the oven to 350°F. Spray an 8x8 inch pan with nonstick spray.
PREPARE THE CRUST:
In a medium bowl, use a hand mixer to beat the butter and sugar together until creamy. Stir in the flour and salt until well combined and crumbly. Press the crust into the bottom of the prepared pan. Bake about 12 minutes until just barely golden around the edges.
PREPARE THE TOPPING:
In the same bowl (no need to wash it) whisk together the sugar, flour, baking powder and salt. Add the egg, egg white and lemon juice and stir until well-blended and smooth.
Sprinkle the cranberries and coconut evenly over the baked crust. Pour the lemon filling over the top. Bake 40 to 45 minutes until golden and set. Cool completely in the pan on a wire rack.
Sprinkle with powdered sugar, if using, before cutting into squares.