Preheat oven to 350F and grease an 8x8 inch baking dish.
Place the chopped dates in a heatproof bowl. Pour the boiling water over them, stir in the baking soda, and let the mixture sit for 15 minutes to soften.
In a separate bowl, cream the butter and brown sugar. Beat in the eggs one at a time, then add the vanilla. Fold in the gluten free flour.
Mash the date mixture slightly with a fork, then fold it (along with its liquid) into the cake batter. Pour into the prepared dish and bake for 30-35 minutes until a toothpick comes out clean. Leave the cake in the dish.
While the cake bakes, make the sauce. Combine the butter, brown sugar, heavy cream, and vanilla in a saucepan over medium heat. Bring to a gentle boil and simmer for 3 minutes until thickened.
Poke holes all over the warm cake and pour half the toffee sauce over the top. Serve warm with the remaining sauce drizzled on individual slices.