Preheat the oven to 180 C / Gas 4. Lightly grease a baking tray.
Place the butter and sugar in a large bowl and mix with electric beaters until smooth. Add the egg yolk and vanilla, then mix in the flour until just incorporated. Spoon the biscuit mixtue onto the greased baking tray, and spread evenly. Top with the beaten egg white. Sprinkle with dark brown sugar and pecans.
Bake until golden and set, about 30 minutes. Cool completely before cutting into squares.