For the cake: Position an oven rack in the center of the oven; preheat the oven to 350 degrees F. Grease a loaf pan with butter and dust with flour; set aside.
Combine the dates, baking soda and 1/2 cup water in a small saucepan over high heat. Bring to a boil, then remove from the heat and set aside.
Place the salt and flour in a medium bowl and whisk to combine; set aside. Combine the butter and brown sugar in a stand mixer fitted with a paddle attachment. Beat on medium speed until light and airy, about 5 minutes, stopping occasionally to scrape down the sides of the bowl. Add the vanilla extract; then beat in the eggs, one at a time, just until incorporated. Remove the bowl from the mixer and stir in the date mixture until well combined. Add the flour mixture and the cranberries, and stir just until incorporated (don't overmix). Pour the batter into the prepared loaf pan. Arrange the reserved cranberries, evenly spaced, on top.
Bake on the center rack of the oven until the center of the cake is set and a cake tester inserted into the middle comes out clean, 45 to 55 minutes. Meanwhile, prepare the toffee sauce.
For the toffee sauce: Melt the butter in a medium saucepan over medium heat. When it foams, add the brown sugar and vanilla extract; stir once. Heat until the mixture is bubbling and has turned the color of maple syrup, about 5 minutes. While stirring, carefully add the cream in a slow stream. Cook until the sauce bubbles up and increases in volume, about 2 minutes. Remove from the heat.
When the cake is finished cooking, remove the pan from the oven and place on a cooling rack to cool. Once the cake is cool, run a knife around the perimeter of the cake and invert the pan to unmold the cake.
Place the cake upright on a serving plate. Serve at room temperature or slightly warmed, covered in warm toffee sauce.