Preheat oven to 350°F. Grease an 8 ½ by 4 ½ inch loaf pan and line with a strip of parchment.
Whisk or sift together flour, baking powder and salt.
In a large mixing bowl, beat butter and sugar at medium-high speed with an electric mixer until light and creamy. Add the egg and beat until fluffy. Scrape down sides of bowl and add 1 teaspoon of the vanilla extract.
With a heavy duty scraper, add flour mixture, and milk alternately, ending with flour mixture. Beat the batter with the mixer for about 30 seconds.
Weight out 7 oz of batter and put it in a separate bowl. Repeat with another 7 oz of batter so that you have three bowls. The original bowl will be the vanilla.
To one of the measured bowls, stir in melted chocolate, cocoa powder and oil to make a chocolate batter. To the other, beat in Jell-o powder and extract (if using). Add remaining ½ teaspoon of vanilla to the vanilla batter to pump the vanilla flavor.
Drop spoonfuls of batter into the pan, alternating colors and creating your own pattern. Repeat until you’ve used all the batter.
Bake the cake at 350 degrees F. for about 45 minutes or until a skewer inserted comes out with moist crumbs.
Let the cake cool in the pan for about 20 minutes, then carefully remove from pan and let it cool completely.