Preheat your oven to 350°F (175°C). Grease and flour a 9x13-inch baking pan.
In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, combine the flour, baking powder, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the milk. Mix until just combined.
Pour the batter into the prepared baking pan and spread it evenly. 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 10 minutes.
While the cake is cooling, dissolve the strawberry gelatin in boiling water, then stir in the cold water.
Poke holes all over the top of the cooled cake using a fork or a skewer. Pour the gelatin mixture evenly over the cake, allowing it to seep into the holes.
Refrigerate the cake for at least 2 hours to set the gelatin.
Once chilled, spread the whipped topping over the top of the cake. Decorate with blueberries and sliced strawberries.
For an extra touch, pipe whipped cream around the edges before serving.