Grease a 1-1⁄2-quart, deep oval baking dish (or one that will fit in your slow cooker) with the butter.
Meanwhile, add all of the sauce ingredients to a heavy, medium-size saucepan, and bring to a boil over medium-high heat. Cook, stirring frequently, until the mixture is thickened and the cranberries are cooked, about 10 minutes (watch, as you don’t want the mixture to boil over). Pour half of this sauce into the baking dish, and place the dish in the freezer until the mixture solidifies. (Refrigerate the remaining sauce, then warm in a small saucepan over medium-low heat when ready to serve.)
Wash out the medium saucepan, then add the dates, dried cranberries, and 1 cup of water, and bring to a low boil over high heat. Remove from the heat and immediately mix in the baking soda (the mixture will foam a bit). In a small bowl, whisk together the flour, baking powder, espresso powder, and salt. In a stand mixer (or a large bowl with a hand mixer), beat the butter and granulated sugar on medium-high speed until light and fluffy, about 5 minutes. Slowly beat in the eggs and vanilla, scraping down the sides of the bowl with a rubber spatula to incorporate everything. Beat on medium-high speed for a couple of minutes to mix well. Reduce the speed to low, then beat in half of the flour mixture, then the date mixture. On low speed, beat in the remaining flour mixture until just combined (do not overmix). Remove the baking dish from the freezer. Pour the batter on top of the solidified cranberry mixture, and cover tightly with foil.
Place a trivet or small rack (or two sticks of foil to form a cross) in the slow cooker, then pour in about 1 inch of boiling water. Set the baking dish on top. Cover and cook on low until a toothpick inserted into the center of the pudding comes out with some moist crumbs, 3 to 4 hours. Let cool to warm, then poke all over with a fork. Pour the reserved sauce over the top, and serve.