PREHEAT oven to 350F. Line an 8-in. square baking dish with parchment, with sides hanging over. Stir 1 cup flour with 1/4 cup granulated sugar in a large bowl. With your fingers or a pastry cutter, work cold butter into flour mixture until coarse crumbs form. Press dough into prepared dish. Bake in centre of oven until golden around edges, 25 to 30 min.
STIR dates and 1/4 cup water in a small saucepan and set over medium-high. Bring to a boil, covered, stirring occasionally. Reduce heat to medium and gently boil 3 min. Remove from heat and stir in baking soda. Set aside to cool.
STIR 4 tsp flour with baking powder and salt in a small bowl. In the same bowl used for the base, whisk eggs. Whisk in 1/2 cup sugar, corn syrup, melted butter and vanilla. Gradually stir in flour mixture. Fold in date mixture and pecans. Pour over hot baked crust and smooth top.
BAKE in centre of oven until filling is set and edges are deep golden, 30 to 35 min. Cool completely on a rack. Cut into 24 bars. Sift icing sugar overtop just before serving if desired. Store in an airtight container and refrigerate up to 1 week or wrap in foil and freeze up to 1 month.