Preheat the oven to 180C/160C fan/gas 4. Butter a 23cm square cake tin (or a 20x28cm rectangular tin) and line it with baking parchment.
In a medium saucepan, combine the chopped dates, coffee, milk, and bicarbonate of soda. Simmer over medium-low heat for 5-7 minutes until the dates have softened and broken down.
In a stand mixer or large bowl, beat the butter and sugar together until light and fluffy. Sift in the gluten-free flour, mixed spice, and xanthan gum (if using). Add the beaten eggs and mix until just combined. Stir in the warm date mixture and mix gently to form a smooth batter.
Pour the batter into the prepared tin and bake for 40 minutes, or until the sponge is risen, firm, and a skewer inserted in the center comes out clean.
While the sponge bakes, make the toffee sauce. Place 150ml of the double cream in a saucepan with the butter, light brown sugar, dark muscovado sugar, and espresso powder. Heat over medium heat until the butter has melted, then simmer for 4-5 minutes to thicken. Remove from heat and stir in the remaining cream.
Let the sponge cool slightly, then cut it into squares or rectangles. Pour warm toffee sauce over the top and serve with a scoop of ice cream or a drizzle of cream.