Preheat oven to 350 degrees F. Grease a 9x13-inch cake pan.
Thaw the frozen strawberries (I did this in the microwave) and mash them very well or put them in a blender until they are liquid. You should end up with 1 cup of liquid.
In a large bowl, blend together cake mix, mashed strawberries, water, melted butter, and eggs for two minutes. Spread the batter into the prepared pan. It will be very thick.
Bake uncovered for 23-28 minutes until a toothpick inserted into the center comes out clean.
While the cake is still hot, poke holes all over the top with a fork.
Spread sweetened condensed milk over the top of the cake. Cool the cake completely then refrigerate for 2 hours.
In a large bowl using a hand mixer, blend together cream cheese, powdered sugar, heavy whipping cream, and vanilla until thick and creamy, about 3-4 minutes.
Spread the topping over the cake then top with sliced strawberries.