Preheat the oven to 350°F (175°C) and prepare a 9x9-inch baking pan. Pour boiling water over the chopped dates, stir in the baking soda, and let the mixture cool. The dates will soften and become almost jam-like.

Beat the butter and brown sugar together until light and fluffy. Add the egg and vanilla, mixing until smooth and creamy.

Whisk together the flour and salt. Alternate adding the flour mixture and cooled dates to the butter mixture, beginning and ending with the dry ingredients. Mix only until combined, then fold in the walnuts.

Spread the batter evenly into the prepared pan. Bake for 30 to 35 minutes, until the cake is golden and a toothpick inserted in the center comes out with a few moist crumbs.

While the cake bakes, combine the brown sugar, butter, and heavy cream in a saucepan. Bring to a boil and cook for 3 minutes, stirring constantly. Remove from the heat and stir in the shredded coconut and vanilla.

Let the cake cool for 5 minutes, then spread the coconut mixture evenly over the surface while the cake is still hot.

Place the cake under the broiler for 1 to 2 minutes. Watch carefully as the topping bubbles and turns golden brown around the edges.

Allow the cake to cool completely on a wire rack. As it cools, the topping becomes lightly crisp while the cake underneath stays moist. Cut into squares and serve.