Preheat oven to 350℉. Spray a 4-inch cake pan and line the bottom with parchment paper.
You will need 1/2 cup of guava concentrate for the complete recipe. (Cake and gel). You need to strain out the tiny pieces of fruit from the guava juice if there are any.
Then, over medium-high heat in a small saucepan, add 1 cup of juice. Cook for about 5 minutes, or until reduced to 1/2 cup. Whisk often. Set aside to cool.
In a 1-cup liquid measuring cup, whisk the flour, baking powder, and salt. Set aside.
In a small bowl, combine 3 Tbsp guava concentrate, milk, vanilla, and food coloring. Set aside.
In a 2-cup liquid measuring cup, add the butter and sugar. Cream together for about a minute or until fluffy using the hand-held immersion blender with whisk attachment. (See Notes)
Add the oil and egg white. Whisk for about 30 seconds, or until well mixed.
Starting with dry ingredients, add 1/3 and mix using the hand-held immersion blender with whisk attachment until combined. Add 1/2 of the guava mixture and mix. Add 1/3 of the dry ingredients and mix. Add the remaining guava mixture and mix. Add the remaining dry ingredients and mix. Cake batter should be combined, but not overmixed.
Pour cake batter into your 4-inch prepared cake pan. Place on a small cookie sheet or roasting pan. You are less likely to burn yourself using a pan under the cake pan.
Bake for 30-35 minutes. A toothpick should come out clean. This part is tricky: Look closely, it’s not easy to see. It should be dry.
Cool for 10 minutes, remove from the pan, and let cool completely on a wire rack.