Preheat your oven to 350°F (175°C). Grease a 9-inch round cake pan thoroughly with butter or oil, then lightly dust with cocoa powder to prevent sticking. Set aside.
In a large bowl, sift together the all-purpose flour, cocoa powder, baking powder, baking soda, and salt.
In a separate bowl, whisk together the granulated sugar, vegetable oil, eggs, and vanilla extract until smooth. Add the buttermilk and mix well.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined. Slowly pour in the boiling water while stirring to create a thin, smooth batter.
In a heatproof bowl, melt the chocolate and butter together using a double boiler or microwave in short bursts. Stir until smooth. Remove from heat and whisk in powdered sugar until thick and fudgy.
Pour half of the cake batter into the prepared pan. Spoon the fudge filling evenly over the batter, creating a “tunnel” in the center. Top with the remaining batter, smoothing the surface with a spatula.
Bake for 35-40 minutes or until a toothpick inserted into the outer cake (not the fudge center) comes out clean. The edges should be set but the center slightly gooey.
Allow the cake to cool in the pan for at least 15 minutes before slicing. The fudge tunnel will be molten and luscious.
