Prepare the cake batter by boiling chopped dates with water, then mix with baking soda and let sit for 10 minutes.
In a medium bowl, whisk together gluten-free flour blend, baking powder, salt, cinnamon, and nutmeg.
Cream together softened butter and brown sugar until light and fluffy, then add eggs and vanilla extract.
Fold in the date mixture and gradually add the dry ingredients until just combined.
Preheat oven to 350°F (175°C) and grease an 8-inch square baking dish.
Pour the batter into the prepared dish and bake for 30-35 minutes, checking for doneness with a toothpick.
Prepare the toffee sauce by combining brown sugar, heavy cream, and butter in a saucepan, then boil and simmer for 5 minutes.
Drizzle the warm toffee sauce over the baked cake and serve.