Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, and salt. Mix well.
Add the milk, vegetable oil, eggs, and vanilla extract to the dry ingredients. Beat on medium speed for about 2 minutes until well combined.
Carefully stir in the boiling water until the batter is smooth (the batter will be thin).
Pour the batter into the prepared baking pan and bake for 30-35 minutes, or until a toothpick inserted in the center comes out clean.
Once baked, remove the cake from the oven and let it cool in the pan for about 15 minutes.
Using the handle of a wooden spoon, poke holes all over the top of the cake.
In a separate bowl, mix the sweetened condensed milk with the strawberry gelatin until dissolved. Pour this mixture evenly over the cake, making sure it seeps into the holes.
Refrigerate the cake for at least 2 hours to allow the flavors to meld.
After chilling, prepare the blueberry gelatin according to package instructions and pour it over the cake. Refrigerate again for at least 1 hour.
Spread the whipped topping over the top of the cake. Decorate with fresh strawberries and blueberries to create a patriotic design.