Sounds like it was in the oven a little too long! Dark exterior and dry interior are two classic signs of an over-baked cake. When following recipes it’s important to remember that the baking times are not always exact as individual ovens may vary. So be sure to check on your cake intermittently while it’s baking. Remember, if you stick a knife in it and the blade comes out dry/without any wet cake mix on it, it’s ready to come out of the oven. Also remember that your cake will keep baking while it cools down.