In a bowl, whisk the flour, salt and baking soda together until well combined

Set aside until needed

In another bowl, cream the butter and sugar with a whisk until it comes together. You don't need an electric beater, just do this step by hand until the butter and sugar are well incorporated

Add the egg to this and beat again well until the mixture is a bit fluffy

Now add water and cocoa to this and mix well until the cocoa powder has fully blended in

Add the vinegar or lemon juice and the vanilla, mix again

Add milk and mix gently again

Now add the flour mixture in two additions, folding gently after each addition. Do not overmix at this stage, just mix until you don't see any traces of flour remaining and the batter is lump free (small lumps should be okay, they will disappear on baking)

Grease a small 6" cake pan with butter and pour the batter in

Tap the cake pan firmly on the kitchen counter a couple of times to remove any air bubbles in the batter

Preheat the pressure cooker by placing the plate or tray at the bottom of the pressure pan and closing the lid (no water needed, no gasket needed on the lid)

Heat on medium flame for about 2 minutes until the cooker is hot

Gently lower the cake pan with a pot holder into the pressure pan

Close the lid and keep the flame very low (simmer)

Cook for about 45-50 minutes until the a toothpick inserted into the cake comes out clean. The cake will also leave the sides of the pan, which is a good indication that it's cooked

Remove from inside the cooker and cool for 10 minutes in the cake pan

Then, gently invert onto a cooling rack or plate and cool the cake completely before cutting and serving

You can also use this cake as a base and frost it, make multiple layers, etc