Grease a 12-cup muffin pan or line with 12 muffin paper liners.
Sift together the flour, baking powder, and salt into a large bowl. Stir in the sugar.
Lightly beat the eggs in a large bowl, then beat in the milk, oil and vanilla extract.
Make a well in the center of the dry ingredients and pour in the beaten liquid ingredients. Stir gently until just combined; do not overmix.
Divide the batter between 2 bowls. Sift the cocoa into one bowl and mix together. Using teaspoons, spoon the batters into the prepared muffin pan, alternating the chocolate batter and the plain batter.
Bake in the preheated oven for about 25 minutes, until well risen and firm to the touch.
Let the muffins cool in the pan for 5 minutes, then serve warm or transfer to a wire rack and let cool completely.