The slow cooker is a perfect fit for this pudding-like cake: The gentle, moist heat keeps the cake nice and supple until it’s doused at the end with nutmeg-laced toffee sauce. Make the cake a day ahead, and store it in the slow cooker (the fridge is quite full enough around the holidays!).
Bring the dates and milk to a simmer in a small saucepan over medium heat, and simmer until the dates begin to soften, 3 to 4 minutes. Remove from the heat, and stir in the baking soda (it will foam a bit); set aside to cool slightly.
Whisk together the flour, baking powder, salt and pumpkin pie spice in a medium bowl until combined; set aside.
Whisk together the sugar, butter, eggs and zest in a large bowl until smooth. Whisk in the date mixture until just combined. Stir in the flour mixture until just combined.
Line the insert of a 6-quart slow cooker with a double layer of aluminum foil, pressing it smoothly into the corners, and spray well with cooking spray. Pour the batter into the foil, and turn the cooker on low for 4 hours. Fold a double layer of paper towels, and place just under the lid across the pot to absorb the excess moisture and form a tight seal. Cover tightly with the lid. Cook until the middle of the cake has set and a toothpick inserted in the center comes out clean. Let cool slightly, 15 to 20 minutes.
While the pudding cools, bring the sugar, cream and butter to a boil in a small saucepan over medium-high heat; cook, stirring often, until smooth, about 5 minutes. Remove from the heat, and add the nutmeg.
To assemble: Poke holes in the cake with a skewer, and pour half the toffee sauce over it. Let the cake absorb some of the sauce, then pour the remaining sauce over the cake. Let the cake cool, 10 to 15 minutes. Serve with ice cream or whipped cream.