Preheat your oven to 350°F (175°C). Grease and flour a 6-inch cake pan.
In a mixing bowl, cream together the butter and sugar until light and fluffy.
Add the egg and mix well.
In another bowl, whisk together the flour, baking powder, and salt.
Gradually add the dry ingredients to the wet mixture, alternating with the milk. Mix until just combined.
Fold in the lemon and orange zests.
Pour the batter into the prepared cake pan and smooth the top.
Bake for 25 minutes, or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.