Preheat oven to 325 F.
Grease a 9"x5" loaf pan. Line the bottom with parchment paper and grease the paper.
In a large bowl, beat the butter and sugar until creamy.
Add the eggs, one at a time, beating well between each addition
Beat in the butter essence, 2 Tbsp milk, vanilla, and salt.
Stir in the flour and baking powder just until combined.
Spread half the batter in the prepared loaf tin.
Add the remaining 1 Tbsp milk and cocoa powder to the remaining batter until well blended.
Pour the chocolate batter on top of the vanilla batter. Swirl with a butterknife.
Bake on the centre rack for 75 minutes or until a skewer inserted in centre comes out clean.
Let cool in pan for 30 minutes before turning out onto a wire rack and let the cake cool completely.
The cake can be baked up to 2 days ahead; wrap in plastic and keep at room temperature. Or, you can wrap it in plastic and freeze for up to 6 months.