Step 1: Preheat your oven to 350°F (175°C). Grease and flour a bundt pan or a 9-inch round cake pan thoroughly.
Step 2: In a medium-sized bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt until nicely combined. Set aside.
Step 3: In a large mixing bowl, cream together the softened unsalted butter and granulated sugar with an electric mixer until light and fluffy. Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Step 4: Alternately add the dry ingredients mixture and the buttermilk into the butter-sugar-egg batter. Start with about a third of the dry, mix gently, then half the buttermilk, mix, another third of the dry, mix, the remaining buttermilk, mix, and finally the last of the dry ingredients. Mix just until everything is smooth and combined, being careful not to overmix.
Step 5: Divide the batter evenly into three separate bowls. Leave one bowl plain (vanilla). To the second bowl, add the cocoa powder and black food coloring and mix until deep black. To the third bowl, add orange food coloring and mix until vibrant orange.
Step 6: Take spoonfuls of each colored batter and dollop them randomly into your prepared cake pan, alternating colors. Once all the batter is in, gently swirl through the batters once or twice with a knife or skewer to create a marbled effect, without fully mixing the colors.
Step 7: (Optional) Sprinkle the semi-sweet chocolate chips evenly over the top of the cake batter.
Step 8: Bake for about 40–45 minutes, or until a toothpick inserted into the center comes out clean.
Step 9: Let the cake cool in the pan for about 15 minutes before gently transferring it to a wire rack to cool completely.