Step 1: Prepare Your Pan and Oven
Preheat your oven to 350°F (175°C). Thoroughly grease a 9×5-inch loaf pan with butter or cooking spray, then dust lightly with flour (tap out excess). For extra insurance against sticking, line the bottom with parchment paper. This prep work ensures your beautiful cake releases perfectly!

Step 2: Cream Butter and Sugar
In a large mixing bowl, beat the softened butter and 1¾ cups sugar together with an electric mixer on medium-high speed for 4-5 minutes. The mixture should become light, fluffy, and pale in color. This step incorporates air into the batter, which creates that light, tender crumb, so don’t rush it!

Step 3: Add Eggs One at a Time
Reduce mixer speed to medium and add eggs one at a time, beating well after each addition (about 30 seconds per egg). Make sure each egg is fully incorporated before adding the next. The mixture should look smooth and glossy.

Step 4: Combine Dry Ingredients
In a separate bowl, whisk together the flour, baking powder, and salt. This ensures the leavening agent is evenly distributed throughout your cake.

Step 5: Alternate Adding Wet and Dry
With the mixer on low speed, add the flour mixture in three additions, alternating with the milk (begin and end with flour). The pattern is: flour, milk, flour, milk, flour. Mix just until combined after each addition—don’t overmix! Stir in the vanilla extract until just incorporated.

Step 6: Prepare the Chocolate Batter
In a small bowl, whisk together the cocoa powder, hot water (or coffee), and 1 tablespoon sugar until smooth and completely lump-free. Let it cool slightly. Transfer about ⅓ of the vanilla batter (roughly 2 cups) to another bowl. Fold the chocolate mixture into this portion until fully combined and evenly colored.

Step 7: Create the Marble Effect
Spoon alternating dollops of vanilla and chocolate batter into your prepared pan. Start with vanilla, add chocolate, then vanilla, and so on, layering them randomly throughout the pan. Once all batter is in the pan, use a butter knife or skewer to create swirls. Insert the knife all the way to the bottom and make figure-eight patterns or S-curves through the batter 5-6 times. Don’t overdo it—you want distinct swirls, not a muddy mixture!

Step 8: Bake to Perfection
Place the pan in the preheated oven and bake for 50-60 minutes (bundt pan: 45-50 minutes). The cake is done when a toothpick inserted into the center comes out clean or with just a few moist crumbs, and the top springs back when lightly touched. The top should be golden brown.

Step 9: Cool the Cake
Let the cake cool in the pan on a wire rack for 15 minutes. This cooling time allows the structure to set so it won’t fall apart. Run a knife around the edges, then carefully invert onto the wire rack. If you used parchment paper, peel it off. Let the cake cool completely before slicing or glazing.

Step 10: Optional Glaze
If using the chocolate glaze, whisk together powdered sugar, cocoa powder, milk, and vanilla until smooth. The consistency should be thick but pourable—add more milk if needed. Drizzle over the cooled cake and let set for 10 minutes before serving.