Preheat oven to 350 degrees F. Spray a 9x13 baking pan with nonstick cooking spray. Set aside.
In a large mixing bowl, cream together butter and sugar. Mix together eggs, vanilla extract, and milk then add flour, 1 cup at a time, baking powder, and salt. Mix until all is well combined. The batter should not be drippy, it will be thick yet spreadable.
Into the prepared baking pan, add half of the cake batter, making sure it is evenly spread across covering the entire bottom of the pan.
Next, add the slices of guava paste on top of the cake batter in the square pan, making sure to cover the entire cake layer. Add the remaining cake batter on top of the guava slices, making sure to cover all the guava slices.
Bake for 45-50 minutes until brown on top. Once completely cooled, cut the square cake into rectangular cake bars. Serve and enjoy!