Step 1: Prepare the oven and baking dish. Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.
Step 2: Mix the cake batter. In a large bowl, combine the cake mix, 1 cup of full-fat coconut milk, vegetable oil, eggs, and optional coconut extract. Beat with an electric mixer on medium speed for 2 minutes, until smooth.
Step 3: Bake the cake. Pour the batter into the prepared baking pan. Bake for 30-35 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 4: Prepare the coconut soak. While the cake is baking, combine the remaining coconut milk (from the can, about 1/2 cup), granulated sugar, and vanilla extract in a small saucepan. Heat over medium-low heat, stirring until the sugar is dissolved. Do not boil.
Step 5: Soak the warm cake. As soon as the cake comes out of the oven, use a fork or skewer to poke holes all over the top of the warm cake. Slowly pour the warm coconut soak evenly over the entire cake, ensuring it seeps into the holes.
Step 6: Chill the cake. Let the cake cool completely on a wire rack, then cover and refrigerate for at least 4 hours, or ideally overnight, to allow the cake to absorb the soak and become incredibly moist.
Step 7: Decorate and serve. Once chilled, spread the thawed whipped topping evenly over the top of the cake. Garnish with toasted sweetened flaked coconut, if desired. Serve cold.