Preheat your oven to 180C / 350F / 160C fan. Grease an (approximately) 8 inch pie dish with butter.
In a bowl, sift together the flour, cocoa, baking powder and salt. Mix well to combine.
In a separate bowl or the bowl of a stand mixer, beat together the melted butter and sugar until smooth. Add the egg and vanilla and beat well, scraping down the sides of the bowl as necessary. The mixture should look smooth before moving on.
Add 1/3 of the flour mixture to the butter mixture. Stir through gently until just combined. Now add half of the milk, Mix gently again. Continue like this until all the flour and milk is combined… just. It is important not to overmix or be too heavy handed otherwise the cake may turn out dense and tough.
Pour the batter into your prepared pie dish
For the chocolate sauce
Mix together the cocoa and sugar in a small bowl. Sprinkle it evenly over the top of the cake batter
With an upturned spoon in one hand, slowly dribble the boiling water over it, moving it around so the water is dribbled evenly over the whole cake (see notes)
Bake the pudding for 25-30 minutes. The top should look dry and be a little springy.
Pouring the boiling water over the back of a spoon very slowly will allow it to sit on top of the cake batter and not create holes in your cake batter.
For best results you should always weigh ingredients like flour and sugar. Kitchen scales are relatively cheap but if you can’t weigh the ingredients, use the spoon and level method (don’t scoop).