Preheat your oven to 325°F (163°C) and grease your loaf pan.
In a sizable mixing bowl, blend the butter and sugar until the mixture is airy and light.
Add the eggs sequentially, fully mixing each into the batter before introducing the next.
In another bowl, sift together the flour, baking powder, and salt.
Alternate adding the dry mixture and the milk to the creamy mixture, beginning and ending with the dry ingredients.
Stir in the Hennessy and vanilla extract until just combined.
Transfer the batter into the greased loaf pan, evening out the surface with a spatula.
Allow the cake to bake for 60 to 70 minutes, or until a skewer comes out clean when inserted in the middle.
After baking, let the cake sit in the pan for 10 minutes, then move it to a cooling rack to cool off entirely.