Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. Set aside.

In a large mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. This process can take around 3-4 minutes using an electric mixer.

Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.

Gradually add the dry ingredient mixture to the butter mixture, alternating with the milk. Begin and end with the dry ingredients, mixing well after each addition. Mix until the batter is smooth and well combined.

Pour the batter into the prepared cake pan and spread it evenly. Bake in the preheated oven for approximately 25-30 minutes or until a toothpick inserted into the center comes out clean.

While the cake is baking, prepare the guava filling. In a small saucepan, combine the guava paste and water. Cook over low heat, stirring constantly until the guava paste has melted and the mixture has a smooth consistency. Remove from heat and set aside to cool slightly.

Once the cake is baked, remove it from the oven and let it cool in the pan for 10 minutes. Then, transfer it to a wire rack to cool completely.

When the cake has cooled, carefully slice it horizontally into two equal layers.

Spread the guava filling evenly over the bottom layer of the cake, leaving a small border around the edges. Place the top layer of the cake over the filling.

Now, let's prepare the meringue frosting. In a heatproof bowl, combine the egg whites, granulated sugar, and water. Place the bowl over a pot of simmering water, ensuring the bottom of the bowl doesn't touch the water.

Whisk the mixture continuously until the sugar has dissolved and the mixture is warm to the touch. Remove from heat and transfer the mixture to a mixing bowl.

Using an electric mixer, beat the mixture on high speed until stiff peaks form and the frosting is glossy. This usually takes around 5-7 minutes. Stir in the vanilla extract.

Frost the entire cake with the meringue frosting, ensuring a smooth and even coating. You can use a spatula or a piping bag for a decorative touch.

Optional: For an extra touch, you can lightly toast the meringue frosting using a kitchen torch.

Let the cake sit for a few hours before serving, allowing the flavors to meld together. Slice into wedges and serve this delectable Puerto Rican Panetela to your loved ones.