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1
Preheat the oven to 350F.
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2
Generously butter a 9-by-5-inch loaf pan; set aside.
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3
Whisk together the cake flour, baking powder, and salt; set aside.
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4
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter and sugar until light and fluffy, about 5 minutes.
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5
Add the eggs one at a time, beating until combined after each addition and scraping down the sides of the bowl as needed.
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6
Mix in the vanilla.
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7
Add the flour mixture in two batches, alternating with the buttermilk and beginning and ending with the flour.
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8
Set aside one-third of the batter.
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9
In a bowl, mix the cocoa and the boiling water with a rubber spatula until smooth.
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10
Add the cocoa mixture to the reserved cake batter; stir until well combined.
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11
Spoon the batters into the prepared pan in two layers, alternating spoonfuls of vanilla and chocolate to simulate a checkerboard.
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12
To create marbling, run a table knife (or wooden skewer) through the batters in a swirling motion.
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13
Bake, rotating the pan halfway through, until a cake tester comes out clean, 40 to 50 minutes.
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14
Transfer pan to a rack to cool 10 minutes.
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15
Turn out cake from the pan and cool completely on the rack.
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16
Pour glaze over cake, letting it drip down the sides.
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17
Cake can be kept in an airtight container at room temperature for up to 3 days.
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18
In a small bowl, whisk together the confectioners sugar and milk.The mixture should have the consistency of thin sour cream.
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19
Add the melted chocolate and whisk until glaze is smooth.
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20
If it is too thin, add more sugar, 1 tablespoon at a time; if too thick, add more milk, 1 teaspoon at a time.
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21
Use immediately.
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22
Spoonfuls of vanilla and chocolate batters are alternated, in two layers, in a checkerboard pattern.
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23
A table knife is run through the batters to create swirls.
