Preheat oven to 350 degrees. Grease a 12x12 baking tray using non-stick spray. **See Notes**
Cut guava paste into slices and set aside.
In a large mixing bowl or a stand mixer beat butter and sugar on high speed until light and fluffy.
Add eggs one at a time. Beating batter just to combine. Do not overmix.
Add vanilla extract. Then add flour alternating with milk. Starting with flour.
Pour one equal part of the batter into the prepared pan.
Add guava pieces until you cover the whole layer. (See video for reference)
Pour the other part of the cake batter and cover the guava pieces completely. Use an onset spatula to spread the batter carefully.
Bake for 25-35 minutes or until a toothpick comes out clean when inserted through the center of the cake.
After 10 minutes carefully remove it from the pan by flipping it over another plate or pulling up the parchment paper hanging on the sides.
Let it cool down completely, around 40 minutes before dusting with plenty of confectioner’s sugar on top and cutting.
Cut into evenly squares or as you like it.