Preheat oven to 175°C (350°F). Grease and line a rectangular tray pan (about 9×13 inch / 23x33cm).
In a bowl, whisk together flour, baking powder, and salt.
In another bowl, cream butter and sugar until pale and fluffy. Add eggs one at a time, mixing well after each. Stir in vanilla.
Add dry ingredients alternately with milk, mixing gently until just combined. Do not overmix.
Divide the batter in two. Leave one half plain vanilla. In the other half, mix cocoa powder dissolved in hot water.
Spoon the vanilla and chocolate batters alternately into the tray pan. Use a skewer or knife to gently swirl the two batters together for a marble effect.
Bake for 35–45 minutes, or until a toothpick inserted in the center comes out clean.
Cool completely in the pan, then slice and serve.
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