Preheat the oven to 350°F.
Line a 9 by 13 inch baking pan with parchment paper – I used three 7 x 2 inch round pans.
In a medium bowl, combine the cake flour, baking powder and kosher salt, whisking until combined.
In a stand mixer fitted with the paddle attachment, combine the butter and sugar and beat on medium speed until well combined, about 3 minutes.
Add the egg whites, in three additions, beating each addition until well combined, about 1 minute.
Add the oil and mix until combined, about 1 minute or more.
In a separate bowl, whisk together the milk, vanilla, guava purée and food colouring.
On low speed, alternate between adding the wet and dry ingredients, mixing each until combined before adding the next addition.
Pour the mixture into the parchment lined pan(s) and bake until a toothpick inserted into the center comes out clean – 25-30 minutes for the one pan, 18-20 minutes for the three round pans. Let the cake cool in the pan for 10-15 minutes before transferring to a wire rack to cool completely.