Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking dish.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, and salt.
Add the softened butter, eggs, vanilla extract, and milk to the dry ingredients. Mix until just combined and the batter is smooth.
Spread half of the batter evenly into the prepared baking dish.
Evenly distribute the guava paste cubes over the batter.
In a small bowl, mix the ground cinnamon and brown sugar together. Sprinkle half of this mixture over the guava layer.
Spread the remaining batter over the guava and cinnamon layer, smoothing it out gently.
Sprinkle the remaining cinnamon and brown sugar mixture on top of the batter.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for about 10 minutes before slicing and serving.