Generously grease a 1-quart heatproof pudding basin or a deep, heat-safe glass bowl with butter. Set up a large pot with a steaming rack inside, add enough water to reach halfway up the sides of your basin, and bring it to a gentle simmer.

In a large mixing bowl, whisk together the all-purpose flour, baking powder, and salt until well combined. Stir in the shredded suet, granulated sugar, dried currants, and lemon zest, ensuring the currants are evenly coated with the flour mixture.

Create a well in the center of the dry ingredients and pour in the whole milk and vanilla extract. Use a wooden spoon to gently mix everything together until a soft, slightly sticky dough forms.

Transfer the dough into the prepared pudding basin, pressing it down lightly to create an even top. Leave about an inch of space at the top of the basin to allow the pudding to expand as it cooks.

Cover the basin with a pleated layer of parchment paper, followed by a pleated layer of aluminum foil, and tie it securely around the rim with kitchen string. Place the basin into the simmering steamer, cover the pot tightly, and steam for 1 hour and 30 minutes, checking occasionally to add more boiling water if the pot runs dry.

Carefully remove the pudding basin from the steamer using tongs or oven mitts and let it rest for 5 minutes. Remove the foil and paper, invert the pudding onto a warm serving plate, and serve immediately with hot vanilla custard.