Pit and roughly chop the dates and place them in a heatproof bowl.  Pour boiling water over them and stir in the baking soda.  Cover the bowl and set it aside for 30 minutes.

Meanwhile, heat your oven to 350°F (175°C).  Grease a 9×13-inch baking pan. To be extra safe you can line the bottom with a fitted rectangle of parchment paper.

To make the cake batter, blend date-water mixture in a blender or food processor until smooth.  Set aside.

In a large bowl, combine the melted butter and sugars. Whisk in the eggs, then salt.

Using a spoon (trust me, the whisk doesn't work as well), stir in the flour, then date puree. 

Pour the batter into the prepared pan and bake for 25 to 30 minutes, until a toothpick inserted into the center comes out clean (it may have some stray crumbs).  Let it cool in the pan (and we just serve it from the pan as well).  

If possible, make the sauce right before you're ready to serve it.  In a medium saucepan, combine the butter, cream, sugar and vanilla over medium heat and bring to a simmer (but not boil). Whisk for about 10 minutes, until the mixture thickens slightly.

To serve, cut the cake into squares (still warm is just fine). Liberally drizzle each slice with toffee sauce, a pinch of sea salt, and top with a dollop of whipped cream.  A mint leaf or two is lovely but not compulsory.