Start by preheating your oven to 350°F (175°C). Make sure to grease and flour a 9x13 inch baking pan. This prevents sticking and ensures a beautifully baked cake.

In a large bowl, combine the crushed pineapple (with its juice), sweetened condensed milk, beaten eggs, and melted butter.

Stir well to mix all those delicious flavors together. The pineapple juice will infuse moisture into your cake.

In another bowl, mix the all-purpose flour, baking powder, salt, sugar (reserve ½ cup for later), and ground cinnamon. Whisk until even. This helps to evenly distribute the baking powder and cinnamon.

Gradually add the dry blend into the wet mixture. Stir gently until just combined. Be careful not to over-mix; this can lead to a dense cake. As soon as the flour is incorporated, it’s time to stop.

If you're using walnuts, fold them into the batter now. Then, pour the cake batter into your prepared baking pan, spreading it out evenly.

Now, it’s time to pop that pan into the oven. Bake for about 30-35 minutes, or until a toothpick inserted in the center comes out clean. Keep an eye on it; ovens can vary.

Once baked, remove your cake and let it cool in the pan for about 10 minutes. Then, transfer it to a wire rack to cool completely. Slice into squares and serve. Enjoy your masterpiece with a cup of coffee or tea!