Preheat your oven to 350°F (175°C). Grease and line a 9x5-inch (23x13 cm) loaf pan with parchment paper.
Whisk wet ingredients: In a large bowl, beat the sugar and eggs until light. Add the oil, buttermilk, and vanilla extract. Mix until well combined.
Combine dry ingredients: In another bowl, sift together the flour, cocoa powder, baking powder, baking soda, and salt.
Mix batter: Gradually add the dry mixture to the wet, stirring until smooth. Don’t overmix. Fold in chocolate chips if using.
Bake: Pour the batter into the prepared loaf pan. Bake for 50–60 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool: Let the cake cool in the pan for 10–15 minutes, then transfer to a wire rack.
Make the glaze: In a small saucepan or microwave, melt chocolate with milk and butter. Stir until smooth. Pour over the cooled cake.