Butter and flour a 22x12x7 cm (9 x 5 x 3-inch )loaf pan; set aside.
Position a rack in center of oven. Preheat oven to 175C / 350 F degrees.
Sift the flour, baking powder and baking soda in a large bowl.
In a medium bowl, whisk together cacao powder, instant coffee and hot water until completely smooth. Set aside.
Mix the milk, oil, salt and sugars together in a medium bowl. Add the lemon juice and whisk to combine.
Pour the wet ingredients over the flour mixture and whisk until just combined.
Pour one third of the batter into the cacao mixture bowl and whisk until just combined.
Add lemon zest and vanilla to the remaining batter.
Drop 1/4 cup of plain batter into the prepared pan, then dollop with 2 tablespoons of the chocolate batter. Continue layering both batters.
To create marbling, run a sharp knife through the batter in a swirling motion (like an "8").
Bake for 45-47 minutes or until a toothpick inserted near the center comes out clean.
Let cool down into the pan for 10 minutes, then transfer to a cooling rack and let cool completely before slicing.
This cake keeps well up to 3 days on the counter, wrapped into cling film or foil.