Preheat your oven to 350°F (175°C) and lightly grease a 9x13-inch baking pan.
In a large bowl, mix together 1 cup of flour, 1/4 cup of sugar, and 1/4 teaspoon of sea salt. Incorporate the cold cubed butter using a pastry cutter or your fingers until the mixture resembles coarse crumbs. Firmly press this crumbly blend into the bottom of the prepared pan to create an even layer for the base.
In a separate bowl, combine the diced rhubarb with 1/2 cup of sugar, 1/4 cup of flour, and a pinch of salt. Toss to ensure the rhubarb is well coated, then set aside to let the flavors meld.
In another mixing bowl, whisk together the eggs, milk, melted butter, and vanilla extract until smooth. Gradually fold this egg mixture into the rhubarb mixture, ensuring everything is well incorporated.
Carefully pour the rhubarb custard mixture over the crust you prepared earlier, spreading it evenly. Bake in the preheated oven for 35-40 minutes, until the custard is set and the top is a lovely golden hue.
Once baked, allow the bars to cool completely in the pan. For an even better texture, refrigerate for at least 2 hours before slicing into squares.