This is such a fun twist on pound cake - the classic vanilla batter is marbled with cocoa batter for a beautiful surprise once you slice into this cake.
Prep: Heat oven to 360°F and line a 4x8 inch loaf pan with nonstick baking parchment.
Make cake batter: In a large mixing bowl, cream butter with an electric mixer until smooth, then add the eggs, sugar and salt and cream until light and fluffy. Beat in milk. Combine flour and baking powder in a separate bowl, then beat into egg mixture on low until smooth.
Divide batter: Place half of the cake batter into the prepared pan. Beat cocoa powder and extra milk into remaining batter in the bowl. Spread batter on top of light batter in prepared pan, then carefully swirl together using a fork (don't swirl too much or you'll completely mix it - see video for my technique).
Bake: Bake around 50-55 minutes, or until a toothpick inserted into the middle comes out just clean. Cool in the pan for 5 minutes, then lift out and place on a cooling rack to cool completely.
Notes
To make a chocolate glaze
Melt 4oz semi-sweet or dark chocolate with 1 tablespoon butter and 2-3 tablespoons icing sugar over medium-low heat until smooth. Place the COMPLETELY COOLED pound cake back into the loaf pan and spread the glaze on top. Let cool and dry at room temperature until glaze is firm.