1/4 cup (1/2 stick) unsalted butter, room temperature, plus more for pan
1 1/2 cups sifted all-purpose flour plus more for pan
1 1/2 cups chopped pitted dates (about 6 ounces)
1 teaspoon baking soda
1 teaspoon baking powder
1/2 teaspoon sea salt
1 cup sugar
1 teaspoon vanilla extract
2 large eggs
10 1/2 ounces dark muscovado sugar
1/2 cup heavy cream
1/4 cup (1/2 stick) unsalted butter
1 tbsp black treacle
1/2 teaspoon vanilla extract
Preheat oven to 350 °. Butter and flour Bundt pan. Bring dates and 1 1/4 cups water to a boil in a medium heavy saucepan with tall sides. Remove from heat; stir in baking soda (mixture will become foamy). Set aside; let cool.
Whisk 1 1/2 cups flour, baking powder, and salt in a small bowl. Using an electric mixer, beat 1/4 cup butter, sugar, and vanilla in a large bowl to blend (mixture will be grainy). Add 1 egg; beat to blend. Add half of flour mixture and half of date mixture; beat to blend. Repeat with remaining 1 egg, flour mixture, and date mixture. Pour batter into mold.
Bake until a tester inserted into center of cake comes out clean, 40 –45 minutes. Let cool in pan on a wire rack for 30 minutes. Invert pudding onto rack. DO AHEAD: Can be made 1 day ahead. Cover and let stand at room temperature.
Melt the butter, muscovado sugar and treacle over a very low heat in a heavy-based saucepan. Once the butter’s melted, stir gently until everything else is melted too. Now stir in the cream, then turn up the heat and bring to a boil. Continue to boil, stirring constantly, for 3 minutes. Remove from heat; stir in vanilla. DO AHEAD: Can be made 4 hours ahead. Let stand at room temperature. Rewarm gently before using.