Heat the oven to 175°C (350°F). Grease and line a 20 cm (8-inch) square pan, or grease a 23 x 13 cm (9 x 5-inch) loaf pan, leaving a little parchment overhang if desired for easy lifting.

Place the finely chopped dates and baking soda in a heatproof bowl. Pour over the boiling water and let the mixture stand for 10 minutes so the dates soften and release their natural sweetness.

After soaking, mash the dates lightly with a fork, keeping some texture rather than making a completely smooth purée. Do not drain the liquid because it is essential to the cake's moist texture.

Beat the softened butter and packed light brown sugar for 2-3 minutes, until creamy and slightly lighter in texture. Scrape the sides of the bowl so the mixture is evenly combined.

Beat in the room-temperature eggs one at a time, mixing well after each addition. Add the vanilla extract and stir until the batter looks smooth.

In a separate bowl, whisk together the flour, baking powder, fine salt, and optional cinnamon. Add these dry ingredients to the butter mixture and stir only until no broad streaks of flour remain.

Fold in the warm date mixture and the chopped walnuts or pecans, if using. Use a gentle hand and stop as soon as the dates and nuts are distributed, since overmixing can make the cake heavy.

Transfer the batter to the prepared pan and smooth the top. Bake a square cake for 35-45 minutes or a loaf for 50-60 minutes, until the center springs back lightly and a skewer comes out with only a few moist crumbs.

Cool the cake in the pan for 15-20 minutes, then transfer it to a rack and cool for at least 20 minutes overall. Slice and serve warm or at room temperature with your preferred topping.