Get the butter out of the fridge and softening. Also start to preheat the oven to about 180 Celcius/350 Farenheit. Butter or oil a 9x13 dish so the parchment will hold. Place the parchment perpendicular to the pan so you can fold over the partchment over the long edge.
With a spoon or preferably a silicone spatula, mix butter, sugar, salt, and vanilla together. Pour flour in waves about 1/4 each time to mix into the butter mixture. When fully mixed, press into the pan on top of the parchment paper. Smooth as even as possible.
Precook the crust for about 20-25 minutes, until edges are slightly brown. Remove and ever so lightly poke holes into soft crust with a fork.
Break eggs into a bowl and whisk. Add 400g of sugar (2 cups) and mix. add 46g flour (about 6 Tbls) and mix. Add grapefruit juice and mix. Add food coloring if so desired. This will be a very runny mix. After mixing well, let sit for a couple of minutes to let air bubbles release.
Pour mixture over crust and back into the oven. It will likely take about 25 minutes, but start checking at 20 to see if it is set but still has a wobble.
When cooked, let cool at room temp for an hour or two. When mostly cooled, place in fridge for at least an hour, or overnight Dust with powdered sugar and serve. An hour and an hour is minimum. More is better.