Prepare your steaming setup by placing a wire rack at the bottom of a large, deep pot. Fill the pot with enough water to reach halfway up the sides of your pudding mold, and bring it to a gentle simmer over medium heat.

Generously grease a 1-quart pudding mold or a heat-proof glass bowl with butter to prevent sticking, and set it aside.

In a large mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, ground cinnamon, and ground ginger until thoroughly combined and free of lumps.

In a separate medium bowl, beat the softened butter and granulated sugar together with a hand mixer until the mixture is light, fluffy, and pale yellow, about 3 minutes. Stir in the molasses until smooth.

Add the dry ingredient mixture and the whole milk to the creamed butter in alternating additions, stirring gently with a wooden spoon or spatula until a thick, uniform batter forms.

Gently fold the raisins into the batter until they are evenly distributed throughout the mixture.

Spoon the batter into the prepared pudding mold, smoothing the top with a spatula and leaving at least 1 inch of headspace at the top for expansion.

Cover the mold tightly with a piece of buttered aluminum foil, pleated in the middle to allow for rising, and secure it firmly around the rim with kitchen twine.

Carefully lower the mold onto the rack in the simmering water, cover the pot with its lid, and steam for 60 minutes, checking occasionally to ensure the water level remains halfway up the mold.

Remove the mold from the pot and let it rest for 10 minutes before carefully removing the foil. Invert the Figgy Duff onto a serving platter, slice into thick rounds, and serve warm with custard or a sweet brown sugar sauce.