Place a steamer rack inside a large stock pot. Add enough water so that when placed on the steamer rack, the pudding mold will be about ¾ of the way submerged. Cover the pot and bring the water to a boil.
Grease the pudding mold well with the softened butter. Add 2 tablespoons of flour and turn the mold to coat evenly. Cut out a 6-inch (12 cm) round of parchment paper and grease with butter.
Grate the frozen butter on the large holes of a box grater into a large bowl.
Add the remaining 3¾ cups (465 G) flour, the salt, and baking powder. Mix with a fork until evenly distributed.
Add the sugar, raisins, currants, and lemon zest. Stir to combine.
Add the milk and eggs. Stir with a rubber spatula until the batter comes together and there are no dry spots remaining, being careful not to overmix.
Transfer to the batter to the prepared pudding mold and smooth the top. Place the parchment round over the batter, greased side down. Secure the lid to the mold.
Carefully lower the pudding mold into the steam bath. Reduce the heat to medium-low. Cover the pot and steam for 80–90 minutes, until the internal temperature of the pudding is at least 180°F (80°C). Add more water as needed to maintain the level.
While the pudding steams, make the crème anglaise: Prepare an ice bath in a large bowl, filling about halfway.
In a medium bowl, whisk together the egg yolks and ¼ cup (50 G) sugar. Set aside.
In a small saucepan, combine the milk, heavy cream, remaining ¼ cup (50 G) of sugar, the salt, and the vanilla bean (if using vanilla extract, stir into the cooled crème anglaise). Cook over medium heat for 2-3 minutes, until beginning to steam. Remove the pan from the heat.
Ladle ½ cup (120 ML) of the milk mixture into the egg yolk mixture and whisk well. This will temper the eggs so they do not curdle when added back to the pot. Pour the egg yolk mixture back into the pot with the remaining milk mixture.
Return the saucepan to medium heat and cook, stirring continuously with a wooden spoon to ensure the egg yolks don’t cook on the bottom of the pot, for about 5 minutes, until the custard coats the back of the spoon.
Set a strainer inside a medium bowl, then set the bowl in the ice bath, making sure the water doesn’t come over the sides. Strain the crème anglaise into the bowl. Whisk until cool, about 5 minutes. Cover and refrigerate until ready to serve.
Once the pudding has cooked through, carefully remove it from the steam rack and let sit for 15 minutes on a cooling rack. Uncover the mold and remove the parchment round. Run a knife around the edge of the pudding to loosen. Carefully flip onto a serving platter.
Slice and serve the spotted dick warm with the crème anglaise alongside.