Preheat the oven to 350°F. Line a square 8x8 inch baking pan with parchment paper, leaving an overhang to make it easier to remove from the pan after it's baked. You can cut slits in the corner to make it fit better in the pan.
To make the shortbread, mix the butter, sugar, vanilla, almond extract and salt together with a rubber spatula. Add the flour and stir until completely combined. Press the shortbread dough into the prepared baking pan. Press it evenly into all four corners, doing your best to make sure it's even and flat. Bake in the preheated oven for 24-26 minutes until the crust is just barely golden brown.
Make the lemon curd 5-10 minutes before the shortbread is done. Sift the sugar and flour together in a large bowl. Add the eggs, lemon juice and lemon zest and whisk well until combined. It will be very liquid, this is what you want because it firms and sets in the oven.
When the crust is done, carefully remove it from the oven and use a fork to poke holes on the surface, making sure not to poke all the way through. With the crust still warm, pour the prepared curd over the crust and carefully place it bake in the oven. Bake for 24-26 minutes or until the filling is set and doesn't jiggle when you give the pan a light tap. Cool completely at room temperature then transfer to the fridge to cool for another hour or so, or overnight.
When they're cooled, use the overhang from the parchment paper to pull the bars out of the pan. Cut into squares and dust with confectioners' sugar. Serve cold or room temperature (I prefer cold).