How to Make It
Make the cake Preheat the oven to 325°. Butter an 8-inch-square baking pan. In a small saucepan, combine the dates with the lemon juice and 3/4 cup of water; cook over moderately low heat, stirring, until the dates are very soft, about 6 minutes. Stir in the currants and let cool.
In a medium bowl, whisk both flours with the baking soda, cinnamon, fine sea salt, cardamom and ginger. In a stand mixer fitted with the paddle, beat the 1 stick plus 6 tablespoons of butter with the brown sugar at medium-high speed until light and fluffy, about 3 minutes. Beat in the eggs one at a time, then beat in the cooled date mixture and the vanilla. At low speed, beat in the dry ingredients until incorporated, scraping down the side and bottom of the bowl as needed.
Scrape the batter into the prepared pan and bake for about 55 minutes, until a toothpick inserted in the center of the cake comes out clean. Transfer the pan to a rack set over a baking sheet.
Meanwhile, make the toffee In a small saucepan, whisk the agave with the brown sugar and fine sea salt and bring just to a boil, stirring to dissolve the sugar. Remove from the heat and stir in the butter.
Gradually spoon half of the warm sticky toffee over the cake, allowing it to be absorbed slightly before adding more. Let cool completely, about 2 hours.
Rewarm the remaining toffee topping if necessary and spread it evenly over the cake. Sprinkle with fleur de sel. Cut the cake into squares and serve.