Preheat oven to 350 degrees F. Grease and line a 9x9" square baking tin.
With an electric beater or stand mixer, cream butter, salt, and powdered sugar for the crust until light and fluffy.
Add in one cup flour and mix until just combined. Do not overwork the dough. Dough may still be crumbly.
Pour dough into baking tin and use the palm of your hand to firmly press it into a crust.
Use a fork to poke some holes in the crust to let steam escape during baking, and bake for 15 minutes or until the edges are just turning golden brown. Remove tin from oven while you mix the filling, which should only take about five minutes.
In a mixing bowl, whisk sugar and flour until well combined. Add in the eggs and yuzu juice and gently stir until combined, trying not to add to much air to your filling.
Pour filling over your still warm crust and place bake in the oven for 20-25 minutes, or until the filling is just set. Filling may still jiggle but should not be runny.
Put bars in the fridge to cool down before dusting and slicing. Warm filling will likely be too gooey to slice. When cool, sift powdered sugar, if using, over the top and slice into squares. Leftovers will keep in an airtight container in the fridge.