Preheat oven to 350 degrees. Generously butter a 9-by-5-inch loaf pan; set aside. Whisk together the cake flour, baking powder, and salt; set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, about 5 minutes. Add eggs, one at a time, beating until combined after each addition and scraping down the sides of the bowl as needed. Mix in vanilla. Add flour mixture in two batches, alternating with the buttermilk and beginning and ending with the flour. Set aside one-third of the batter.
In a bowl, mix cocoa and 1/4 cup plus 2 tablespoons boiling water with a rubber spatula until smooth. Add the cocoa mixture to the reserved cake batter; stir until well combined.
Spoon batters into the prepared pan in two layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard. To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion.
Bake, rotating the pan halfway through, until a cake tester comes out clean, 40 to 50 minutes. Transfer pan to a rack to cool 10 minutes. Turn out cake from pan and cool completely on the rack. Cake can be kept in an airtight container at room temperature up to 3 days.
To make glaze: In a small bowl, whisk together sugar and milk. Whisk in chocolate. Pour over cake, allowing to drip down sides. Serve immediately.