Preheat the oven to 350°. Lightly butter six 1/2-cup ramekins. In a small bowl, whisk the flour with the baking powder, baking soda and salt. In a medium bowl, using an electric mixer, beat the butter with the brown sugar at medium speed until light and fluffy. Beat in the egg and vanilla, then beat in the date puree. At low speed, beat in the dry ingredients. Spoon the batter into the ramekins and smooth the tops. Bake for 20 minutes, or until a toothpick inserted into the centers comes out clean; let cool slightly.