Preheat a convection oven to 160°C / 325°F. Line a half sheet pan with a nonstick rubber mat and spray the borders with nonstick cooking spray.
Bring the water and dates to a boil. Reduce the heat to a simmer and cook for 5 minutes. Remove from the heat and whisk in the baking soda (the mixture will foam). Let steep for at least 20 minutes, then purée with a beurre mixer.
Sift together the flour, baking powder, ginger, and salt.
Cream the butter and sugar in a mixer using a paddle attachment until the butter is creamed (light and fluffy), about 4 minutes. While the machine is running on low speed, gradually add the eggs, one at a time. Scrape the sides of the bowl after each addition.
While the machine is still on, add the dry ingredients in several additions and then the dates. Make sure to occasionally scrape down the sides of the bowl.
Spread evenly onto the prepared sheet pan.
Bake until the cake springs back to the touch at the center of the pan, about 7 minutes.
Let cool to room temperature.
Once the cake has cooled, freeze it for 2 hours before cutting it. It cuts very cleanly when it is partially frozen.
Cut into rectangles 2.5cm / 1in by 12.5cm / 5in by 2.5cm / 1 in.
Reserve in an airtight container at room temperature. Discard after service.