Empanadas with Corn Filling ("Humitas")

Makes 12 small or 8 large empanadas
For the dough:
3 cups flour
1 1/2 sticks butter, chilled and cut into cubes
3/4 teaspoon salt
1 large egg
8 tablespoons cold water
For the filling:
4 cups fresh corn (approximately 1 bag of frozen corn is fine)
3 green onions, thinly sliced
1 tablespoon olive oil
2 cloves garlic, minced
1 teaspoon mustard
3 tablespoons milk
1 small squeeze of lemon juice
1/2 teaspoon smoked paprika
1/2 teaspoon salt
1 teaspoon sugar
For assembly:
1 large egg, whisked
Sea salt or Maldon salt to taste (for garnish)
In a food processor, pulse to combine the flour, butter and salt. Whisk together the egg and the water and add in individual tablespoons until dough just comes together. Press into two discs and cover with plastic wrap. Chill for about 30 minutes to an hour in the fridge.
In a large skillet on medium heat, cook the corn and green onions in olive oil for about 3 minutes. Add garlic, mustard, milk, lemon juice, paprika, salt and sugar, and cook for 5 minutes more until the corn is soft and spices have melded. In a food processor, pulse about 3 or 4 times, until the mixture is homogenous but leaving a few corn kernels whole.
Remove dough from fridge, and roll out on a floured surface as thin as you can, about 1/4 inch or so. Preheat oven to 400°F. Cut round circles using the back of a small saucer (I used a 3-inch for small empanadas and a 6-inch for larger empanadas).
Transfer rounds to a parchment–lined baking sheet. Place 1-2 tablespoons of filling into the center of each round, folding in half and crimping with a fork to enclose the pastry. Brush the tops with egg and dust with a little sea salt or Maldon salt, if you have it. Bake for about 12-15 minutes, until empanadas are golden brown.
Serve immediately with white wine or lemonade.