Preheat your oven to 350°F (175°C). Grease and line the 9x9 inch baking pan with parchment paper, leaving some overhang for easy removal later.
In a large mixing bowl, combine the powdered sugar, unsalted butter, all-purpose flour, and kosher salt. Mix until the mixture resembles coarse crumbs. Press this mixture firmly into the bottom of your prepared pan. Bake for about 15-20 minutes, or until lightly golden. Remove from the oven and set aside to cool slightly.
In another bowl, whisk together the eggs, egg yolk, and granulated sugar until well combined. Add the Meyer lemon juice, lemon zest, and vanilla bean paste. Whisk until smooth.
In a separate bowl, mix together the baking powder and all-purpose flour. Gradually add this dry mixture to the lemon filling, whisking until fully incorporated.
Pour the lemon filling over the pre-baked crust. Return the pan to the oven and bake for an additional 20-25 minutes, or until the filling is set and slightly firm to the touch.
Once baked, remove the bars from the oven and let them cool completely in the pan on a wire rack. Once cool, lift the bars out using the parchment overhang. Cut into squares and dust with sifted powdered sugar before serving.