Preheat your oven to 350°F (175°C). Grease a 9 × 5-inch loaf pan with butter and line it with parchment paper. Leave a little paper overhanging the sides to make removing the baked cake easier.
Place the chopped dates in a heatproof bowl. Pour the boiling water over them and stir in the baking soda. Leave the mixture for 10–15 minutes until the dates soften. Mash a few pieces with a fork if you prefer a more evenly distributed date texture.
In a separate bowl, whisk together the flour, baking powder, cinnamon, and salt. This helps distribute the raising agent evenly and prevents pockets of baking powder in the finished cake.
In a large mixing bowl, beat the softened butter and brown sugar until creamy and slightly fluffy. Add the eggs one at a time, beating well after each addition. Stir in the vanilla extract.If the mixture looks slightly curdled, do not worry; adding the flour gradually should help bring the batter together. Gently stir the cooled date mixture into the butter-and-egg mixture. Mix until the dates are distributed throughout. The batter may look quite moist, which helps produce the loaf's characteristic tender texture.
Add the dry ingredients to the wet mixture and stir gently until just combined. Fold in the chopped walnuts, reserving a few for the top if desired. Avoid overmixing, as this can make the cake heavy.
Spoon the batter into the prepared pan and smooth the surface with a spatula. Sprinkle the reserved walnuts over the top for a rustic finish.
Bake for 50–60 minutes, checking the cake from around 45–50 minutes. If the top becomes too dark before the center is ready, loosely cover it with aluminum foil.Insert a skewer into the center. It should come out without wet batter, although a few moist crumbs are fine. If necessary, continue baking in 5-minute increments. Remove the loaf from the oven and leave it in the pan for about 15 minutes. Carefully lift it out and transfer it to a wire rack to cool completely before slicing.
Cut the cake into thick or medium slices using a serrated knife. Serve plain with tea or coffee, or spread a little butter over each slice. The date flavor often becomes even more pronounced after the loaf rests overnight.