Preheat the oven to 375 degrees F.
Making the cake: Whisk together the flour, sugar, baking powder and salt in a large bowl. In another bowl, whisk together the milk, melted butter, vanilla extract and egg. Fold the wet ingredients into the dry ingredients until just blended. Fold in the dates. Spread the batter evenly in a greased, high-sided, 8-inch square casserole.
Making the sauce: Sprinkle the sugar over the batter. Dot with the butter cubes, and sprinkle the sea salt and orange zest evenly over the top. In a saucepan, combine the orange juice with 1 3/4 cups water. Bring to a boil and pour over the batter.
Bake for 1 hour on the middle rack of the oven. The "pudding" is done when the cake's center springs back when touched and the sauce, which has collected at the bottom of the casserole, is syrupy and bubbling.
Serve warm with a dollop of creme fraiche.