Prepare a 7-inch cake tin by rubbing the inside of the cake pan with softened butter. Keep the cake pan aside for later.
In a medium bowl, whisk together flour, baking soda, salt and spices. Set aside.
In a separate large bowl, or in the bowl of a stand mixer, beat together the butter and the sugar. Do not add the brown sugar yet.
Then, add the egg and beat to combine. Add the molasses or treacle, and whisk together on medium speed.
Then add 1/2 a cup of water, and beat to combine. The mixture will appear curdled. Add the flour-spice mixture and add it in three parts to the mixer. beat gently to combine.
Turn out the batter into the prepared cake pan. Over this sprinkle the brown sugar in an even layer.
Mix together 1 cup hot water with the melted butter and pour this mixture over the brown sugar topping.
In your Instant Pot, add 1,5 cups of water and place a tall trivet.
Place the cake pan over the trivet and close the lid (there is no need to cover the pudding with aluminium foil).
Pressure Cook the batter for 50 minutes. Let the pressure release naturally.
Once cooked, open the lid and let the pudding sit for 5 minutes to let any excess water evaporate.
Serve warm, with a scoop of ice cream or whipped cream.