1. Preheat the oven to 325°. Grease and flour an 8 1/2 x 4 1/2 x 2 1/2 inch loaf pan. Place the pan on an insulated baking sheet or on two stacked regular baking sheets.
2. Whisk together the flour, baking powder and salt.
3. Using a mixer, beat the butter on medium until smooth, about 3 minutes. Add the sugar and beat for another 2 to 3 minutes. Add the eggs, one at a time, beating well after each. Beat in the vanilla. Don't worry if the batter looks curdles, it will still turn out ok.
4. Reduce the mixer speed to low and alternately add the flour mixture in 3 additions and the milk in 2, beginning and ending with the dry ingredients, just until each addition is incorporated.
5. Divide the batter equally into two bowls. Add the flavorings of your choice to each bowl of batter.
6. Dollop spoonfuls of each batter randomly into the prepared pan. Plunge a table knife deep into the batter and zigzag the knife through the batter a few times. Be careful not to over mix the batter as you'll lose the marbled effect and end up with a muddy-looking loaf.
7. Bake the cake for an hour and then check on it. If it looks like it's getting too brown but needs more baking, cover the bread loosely with a tent of foil.
The cake may need up to 30 minutes more to bake, depending on your oven, how dark your baking pan is and what additions you've made. The cake is done when a thin knife or cake tester inserted in the middle comes out clean.
8. Transfer the cake pan to a rack to cool for 15 minutes and then remove it from the pan and continue to cool it (right side up) on the rack until it's room temperature.
This bread is easily customizable to suit your tastes. I've made a basic chocolate and vanilla bread, chocolate and peppermint bread, and a creamsicle loaf. Try one of those combinations, or create your own.
Basic chocolate and vanilla bread: Stir 4 ounces of bittersweet chocolate, melted and cooled, into one half of the batter and keep the other half plain.
Chocolate and Peppermint bread: Stir 4 ounces of high-quality white chocolate, melted and cooled, and 1/4 or 1/2 tsp pure peppermint extract into one half of the batter. Stir 4 ounces bittersweet chocolate, melted and cooled, into the other half.
The recipe originally called for 1/4 tsp of peppermint extract, which creates a very faint peppermint flavor when baked. If you prefer a stronger flavor like I do, add more extract to your taste.
Creamsicle bread: Stir 4 ounces of high-quality white chocolate, melted and cooled, into one half of the batter. Stir the grated zest of one orange, 3/4 tsp orange oil (or orange extract to taste), and a couple of drops of orange food coloring into the other half of the batter.