Preheat your oven to 350 degrees Fahrenheit (175 degrees Celsius) and grease your 9x5 inch loaf pan with butter or non-stick spray.

Dust the greased pan lightly with flour and tap out any excess.

In a large bowl, beat the softened butter and sugar together with an electric mixer on medium-high speed for about 3 to 4 minutes until pale and fluffy.

Add the eggs one at a time, beating well after each addition.

Mix in the vanilla extract.

Alternate adding flour and milk: add one-third of the flour, mix until combined, add half the milk, mix, then repeat with the flour and remaining milk.

Mix until just combined and set the batter aside.

In a second bowl, whisk together the flour, cocoa powder, baking soda, and salt.

Beat the butter and sugar until light and fluffy for 3 to 4 minutes.

Add the eggs one at a time, mixing well after each addition.

Mix in the vanilla extract, red food coloring, and white vinegar until evenly combined.

Alternate adding the dry mixture and buttermilk, starting and ending with dry mixture, mixing until just combined.

Drop large spoonfuls of vanilla batter into the bottom of the prepared pan, followed by red velvet batter, alternating until both batters are used.

Using a skewer or butter knife, swirl through the batters to create a marbled effect.

Bake for 45 to 50 minutes until the top is set and a skewer inserted comes out clean.

Let the cake cool in the pan for 15 minutes before turning it out onto a wire rack.

In a small bowl, whisk together powdered sugar, milk, and vanilla extract until smooth and pourable.

Drizzle the glaze over the cooled cake before serving.