In a bowl, with a mixer on high speed, whip 2 cups cream with 2 tablespoons each granulated sugar and rum until soft peaks form.
In the bottom of a 4- to 5-quart trifle dish or straight-sided glass bowl, spread a third of the date cake cubes level. Drizzle 1/3 cup toffee sauce evenly over cake. Spoon about 1 cup poached cranberries (including juices) over sauce. Spread about half the whipped cream evenly over cranberries. Repeat with another layer of cake, sauce, cranberries, and whipped cream. Layer remaining third of the cake cubes over the whipped cream, drizzle with 1/3 cup toffee sauce, and spoon remaining cranberries over the top. Cover with plastic wrap and chill at least 8 hours.
In a bowl, with a mixer on high speed, whip remaining 2 cups cream with remaining 2 tablespoons each granulated sugar and rum until soft peaks form. Spread over trifle and garnish with slivered dates.
To serve, scoop portions onto plates. Pass remaining toffee sauce to add to taste.
Put 3/4 cup chopped pitted Medjool dates in a blender or food processor. Pour in 1/2 cup boiling water and let stand for 5 minutes. Whirl until smooth.
In a bowl, with a mixer on medium speed (use paddle attachment with standing mixer), beat 3/4 cup (3/8 lb.) room-temperature butter, 1 cup firmly packed brown sugar, and the date purée until well blended and smooth. Add 4 large eggs, one at a time, beating well after each addition.
In another bowl, stir together 2 cups all-purpose flour, 1 1/2 teaspoons baking powder, 1/2 teaspoon salt, and 1/4 teaspoon baking soda; stir into butter mixture until well blended. Scrape batter into a buttered and floured 9-inch square baking pan.
Bake in a 350° oven until a wooden skewer inserted into center of cake comes out clean, 35 to 40 minutes. Let cake cool in pan on a rack for 10 minutes, then invert onto rack; remove pan and let cake cool completely.
Trim off dark edges of cake; reserve for another use or discard. Cut remaining cake into 1-inch cubes.
Toffee sauce. In a 1- to 2-quart pan over medium heat, stir 1 1/2 cups firmly packed dark brown sugar, 1/4 cup (1/8 lb.) butter, 2 tablespoons light corn syrup, and 1 teaspoon lemon juice until sugar is dissolved and mixture is foamy, about 5 minutes. Whisk in 2 tablespoons vanilla and cook 1 minute longer. Whisk in 1 1/2 cups whipping cream and remove from heat. Makes 2 cups.
Poached cranberries: In a 3- to 4-quart pan over medium-high heat, stir 1 1/2 cups orange juice, 3/4 cup granulated sugar, and 1/2 teaspoon vanilla until sugar is dissolved. Add 12 ounces rinsed and sorted fresh cranberries or thawed frozen ones; simmer gently just until skins begin to split, about 5 minutes. Remove from heat and chill until cool, at least 30 minutes.