Preheat the oven to 325F (160C), and grease one 8, or 10-cup bundt pan. Or line one 9.5x5-inch loaf pan with parchment paper.
Sift the flour, baking powder, and salt in a medium bowl. Melt the chocolate in a microwave-safe bowl using 30-seconds increments.
In a large mixing bowl fitted with the paddle attachment, mix the butter and sugar until light and fluffy, about 3-4 minutes. Scrape the bottom and sides of the bowl two to three times during the mixing process.
Add the eggs, one at a time, with the mixer on medium-high waiting for each egg to be completely incorporated. Scrape the bottom and sides of the bowl.
Turn the mixer off, and use a rubber spatula to mix in the flour mixture in three additions, alternating with the buttermilk and vanilla. Start and finish with the flour.
Measure about 1/3 of the batter into a medium bowl, and fold in the melted chocolate.
Use two cookie scoops (or large spoons) to scoop the two types of batters in a pattern into the prepared pan.
Insert a butter knife into the cake batter and swirl it like you are writing the number eight. Do it once.
Bake for 45-48 minutes, or until a toothpick comes out from the center of the cake with a few moist crumbs.
Remove the cake from the oven and allow the cake to cook in the pan for about 15-20 minutes, then invert it onto a cooling rack.