Preheat oven to 350°F.
In a medium bowl, stir together the butter and sugar until creamy. Add flour and salt and stir until well combined and crumbly.
Press into the bottom of an 8” x 8” pan that has been sprayed with nonstick spray. Bake for 8-10 minutes, until just barely golden around the edges.
In the same bowl (no need to wash it), combine the sugar, flour, baking powder and salt. Add eggs, lemon zest and lemon juice and stir until well blended and smooth.
Sprinkle the rhubarb evenly over the base, and pour the lemon filling over top. Bake for 30 minutes, or until golden and set. Cool completely in the pan on a wire rack before cutting or freezing. Cut into squares or bars, and sprinkle with icing sugar before serving.