Prepare and bake cake according to package directions in a 9x13 pan. Let cool for 10 minutes.
Mix condensed milk with strawberry juice.
Using a fork, pierce the entire surface of the warm cake.
Slowly pour the milk mixture over the cake, allowing it to soak in.
Let cake cool completely, then refrigerate for at least 2 hours.
In a bowl, whip heavy cream with powdered sugar and vanilla until stiff peaks form.
Spread whipped cream over the chilled cake.
Top with sliced strawberries.
Garnish with fresh mint.
Serve chilled.