Butter or grease 2 loaf pans (5 x 8 x 2 in size approximately). Line each pan with parchment paper. Butter the parchment paper as well.

Preheat the oven to 350F. Lower the heat to 325F if using a dark pan.

Sift together the flour, baking powder, baking soda, and salt. Set aside.

Using an electric mixer, cream the butter and sugar until pale and fluffy. Beat in the eggs one at a time, beating well after each addition.

Beat in the lemon zest and juice plus 2 tablespoons of either the rum or maple syrup/almond mixture until well-incorporated.

Gently fold in the dry ingredients. Transfer half of the mixture into another bowl.

Stir the remaining rum or maple syrup/almond mixture and milk with the melted chocolate. Beat until smooth. Blend well with half of the cake mixture. You will now have two cake batters - plain and chocolate.

Alternately add heaping spoonfuls of the chocolate and plain mixtures on the prepared loaf pans until everything is gone. Tap the pans to level the mixture. Using a knife, cut through the mixture using a swirling motion to create the marbled effect.

Bake for about 45 minutes in the preheated oven or until a tester comes out clean.

Transfer the loaves to a wire rack. Let the cakes cool in the pans for about 10 minutes and then turn out onto the wire rack to cool completely. Slice the cakes only after they have cooled completely.

So perfect with coffee or tea.