Preheat oven to 170C.
Grease and line baking tin.
Beat butter, sugar, salt, and vanilla in mixer bowl until light and fluffy.
Add one egg at a time, and allow to beat until combined before adding the next.
Increase speed to high, and beat for 1 minute.
Sift flour and baking powder together.
Alternately beat in milk and flour, until just combined.
Remove 250g of plain batter in another clean mixing bowl, then fold in cocoa paste.
Spoon the two batters alternately into the middle of the prepared baking tin.
Tap the pan on countertop a few times to remove any trapped air bubbles.
Bake on middle rack, at preheated oven at 170c for 50-55mins or until a skewer inserted come out clean.
Allow to cool for 10 minutes in pan before de-moulding.
Cool completely before slicing.