• Preheat the oven to 350°F (180°C). Grease a 9x5-inch loaf pan with non-stick oil spray.
• Combine milk and vinegar in a cup or bowl and let it stand at room temperature for five minutes.
• Sift all-purpose flour, baking powder, baking soda, and salt together in a medium bowl, and whisk well.
• Add melted butter, oil, sugar, and vanilla to a large mixing bowl. Whisk to combine everything well.
• Add dry ingredients and milk mixture to the wet mixture in two parts. Mix gently until just combined. Do not overmix.
• In a small or medium bowl, stir cocoa powder and hot water until well combined and smooth.
• To make the chocolate batter, take one-third of the vanilla batter and add it to the bowl of cocoa mixture. Mix gently until well combined.
• Now spoon vanilla and chocolate batters alternatively into the prepared pan until both the cake batters are used.
• Gently tap the pan to level the top, and for the swirl, run a knife through the batter 3-4 times, lengthwise.
• Bake the cake for 40-50 minutes or until a skewer comes out clean. However, keep an eye on the cake after 40 minutes of baking, as every oven is different.
• If the cake top is browning too quickly in the oven, loosely cover it with aluminum foil as it bakes.
• Remove the cake and transfer the pan to the wire rack. Allow the cake to cool completely in the pan before removing and slicing.

Storing and Freezing Suggestions

• This cake stays well at room temperature for 3-4 days.
• Refrigerate the cake for up to 8-10 days. If the cake is refrigerated, bring it to room temperature before serving.
• If you want to freeze it, let it cool completely. Wrap the loaf tightly with freezer-safe plastic wrap and place it in the freezer bag. Freeze the cake for up to a month. Then, thaw the cake completely before serving.