Preheat your oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
In a large bowl, whisk together the flour, sugar, salt, and baking powder.
In a separate bowl, cream together the softened butter until light and fluffy. Beat in the eggs one at a time, then stir in the mandarin orange juice and zest, and milk.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.
Pour the batter into the prepared baking pan and spread evenly.
Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes before inverting it onto a wire rack to cool completely.
Notes
Serve plain or with a dusting of powdered sugar, or a simple glaze.