Sift the baking powder and flour in a tiny bowl until mixed. Set that aside.

In a larger bowl, beat the sugar, eggs, and vanilla until creamy and pale. Then, add the milk and flavorless canola oil — just enough to combine.

In two batches, slowly add our flour mixture (from Step 1) to your bowl of wet ingredients. Mix on low speed until nicely combined, and do not go overboard.

Mix ¼ cup of milk with dark cocoa powder in a big bowl until smooth.

Pour ⅓ vanilla batter into the cocoa/milk mixture. Mix until you've got two batters — chocolate and vanilla — with similar consistency. If your chocolate batter seems slightly thicker, add more milk until both batters match up.

Grease an 8'' pan with oil, then line it with parchment paper.

Spoon 3 tablespoons of vanilla batter onto the pan's center. Then, layer 3 tablespoons of chocolate batter right on top. Keep repeating until you've used up all the batter. (If short on time: Pour a batter over another and swirl for a clean, marbled effect).

Grab a skewer or knife and draw lines from the outer edge towards the center.

Place the cake in your preheated oven (with lower and upper heat set at 320°F) for about 45 minutes. Check for doneness: a small toothpick poked into the middle comes out mostly clean.

Leave it on a wire rack (out of the oven) for 15-18 minutes, then lift it off the pan. Serve and slice!