Preheat the Oven: Set your oven to 350°F (180°C). Grease a springform pan with butter and dust with flour.
Make the Cake Batter: In a bowl, beat the butter and sugar until creamy. Add eggs one at a time, beating well after each. Mix in the vanilla extract.
Combine Dry Ingredients: In another bowl, mix flour, baking powder, and salt.
Mix Wet and Dry Ingredients: Gradually add the dry ingredients and milk to the butter mixture, alternating between them. Start and end with the dry ingredients. Mix gently to avoid overmixing.
Bake the Cake: Pour the batter into the prepared pan. Bake for 25-30 minutes or until a toothpick inserted comes out clean. Let the cake cool completely in the pan.
Prepare the Jelly: Dissolve the strawberry jelly packet in 1 cup of hot water, stirring until fully dissolved. Add 1 cup of very cold water and stir well.
Whip the Cream: In a separate bowl, whip the heavy cream until thick and fluffy.
Make the Mousse: Once the jelly starts to thicken slightly, gently fold it into the whipped cream until fully combined, creating a smooth mousse.
Assemble the Cake: Pour the mousse over the cooled cake in the pan. Smooth the top with a spatula.
Decorate and Chill: Top with fresh strawberry slices. Refrigerate for at least 2-3 hours until the mousse sets completely.
Serve: Remove from the pan and serve chilled.