Preheat the oven to 350°F/180°C and grease and line three 8/9inch (20-22cm) cake pans with parchment paper.
In a large mixing bowl, whisk together the flour, sugar, baking powder and salt.
In a separate bowl or mixing jug, whisk together the wet ingredients (except for the boiling water).
Pour the wet into the dry ingredients and mix until mostly combined - a few dry clumps are fine.
Pour in the boiling water and whisk until smooth.
Divide the batter between the three pans then place in the oven.
Bake the cake layers for 20-25 minutes or until a skewer inserted comes out clean.
Remove from the oven and allow to cool for 10 minutes then turn out onto wire cooling racks and allow to cool completely.
To make the buttercream, whip the butter in the bowl of a stand mixer fitted with the whisk attachment until almost white and fluffy - approximately 7-8 minutes.
Add the powdered sugar, a few spoonfuls at a time, and mix for another 2-3 minutes until the buttercream is very light and fluffy.
Add the cream, vanilla and pinch of salt and whip again.
Spread a scoop of buttercream onto a cake layer then add half of the dulce de leche, add another cake layer and repeat. Decorate the top and outside of the cake with the remaining buttercream and serve.
Video
Notes
ESSENTIAL TIPS:
Room temperature eggs and milk incorporate better and create smoother batter
Don't overmix the batter.
Oil-based cakes are more forgiving than butter cakes - harder to mess up!
Cool cakes COMPLETELY before frosting or buttercream will melt
Whip butter for full 7-8 minutes for lightest, fluffiest buttercream
Crumb coat (thin first layer of frosting) seals in crumbs for neat final coat
Level layers for professional appearance - use a serrated knife.