Combine the flour, baking soda, and salt in a medium bowl; set aside.
Cream the butter with 1/2 cup granulated sugar and the brown sugar in a large bowl with an electric mixer on medium speed until smooth and creamy, about 2 minutes. Add in the egg and vanilla, and continue mixing until well blended.
On low speed, gradually stir the flour mixture into the creamed ingredients until well incorporated, without over-mixing. Stop the mixer while beating the batter and scrape the bottom and sides of the bowl with a rubber spatula to help mix it properly.
Chill dough in the refrigerator for 1 hour.
Meanwhile, add the chopped dates, remaining 1/4 cup granulated sugar and the water in a small saucepan over medium heat. Bring the mixture to a boil then reduce the heat to low heat and simmer for about 5 minutes, stirring occasionally. Once the dates have cooked down to the consistency of paste, remove the pan from the heat and stir in the chopped pecans. Set aside to cool.
Use a rolling pin to roll the chilled dough out on a lightly floured piece of parchment or wax paper to a 12x8-inch rectangle. Spread the cooled date mixture on the dough to within about 1/2-inch of the edges. Starting at one of the long ends, begin to carefully roll the dough into a log. Wrap the rolled dough in the wax paper and refrigerate for at least an hour or overnight.
When ready to bake, preheat oven to 350 degrees F.
Unwrap the dough, and slice 1/4-inch slices with a sharp knife. Place slices on baking sheets lined with parchment paper or silicone baking mats and bake for 10 to 12 minutes. Remove from the oven and allow to cool on the baking sheets for about 2 minutes before transferring to a wire rack to cool completely.