Step 1: Prepare the Oven and Baking Pan

• Preheat your oven to 350°F (175°C).
• Grease a 9-inch bundt pan or a loaf pan with butter or non-stick spray. Lightly dust with flour, tapping out any excess to ensure the cake releases easily after baking.

Step 2: Mix the Dry Ingredients

• In a medium bowl, sift together the all-purpose flour, baking powder, and salt. Set aside for later use.

Step 3: Cream the Butter and Sugar

• In a large mixing bowl, use a hand mixer or stand mixer to cream the softened butter and sugar together until light and fluffy (about 3-4 minutes). This step is crucial for creating a light texture in the finished cake.

Step 4: Add Eggs and Vanilla

• Beat in the eggs one at a time, making sure each egg is fully incorporated before adding the next.
• Add the vanilla extract and mix well.

Step 5: Alternate Adding Dry Ingredients and Milk

• Gradually add the flour mixture to the butter and egg mixture in three parts, alternating with the milk. Start and end with the dry ingredients, mixing just until combined. Be careful not to overmix.

Step 6: Create the Chocolate Swirl

• Divide the batter in half. Mix the melted dark chocolate into one half of the batter, stirring until fully incorporated.
• Spoon the vanilla and chocolate batters alternately into the prepared pan. Use a knife or skewer to gently swirl the batters together, creating a marbled effect. Be careful not to over-swirl, or you’ll lose the marbled pattern.

Step 7: Bake the Cake

• Bake in the preheated oven for 50-60 minutes, or until a toothpick inserted into the center comes out clean. If using a loaf pan, the baking time may be slightly shorter, so check around the 45-minute mark.
• Once baked, remove the cake from the oven and allow it to cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely.

Step 8: Prepare the Vanilla Glaze

• While the cake is cooling, whisk together the powdered sugar, milk, vanilla extract, and a pinch of salt in a small bowl until smooth.
• If the glaze is too thick, add a little more milk (1 teaspoon at a time) until you reach the desired consistency.

Step 9: Glaze the Cake

• Once the cake has completely cooled, drizzle the vanilla glaze over the top, allowing it to run down the sides for a rustic look.
• Let the glaze set for about 15-20 minutes before slicing and serving.