Preheat the oven to 180°C and brush 3 x 20cm diameter round cake tins with sunflower oil and then line them with baking paper.
In a bowl of an electric mixer fitted with a whisk attachment, add the eggs, granulated sugar, and oil. Beat at medium speed until well combined, about 2 minutes.
Sift together the salt and the flour, and add the dry ingredients to the egg mixture. Beat at medium speed until just combined.
In a jug, combine the milk and vanilla. In a steady stream with the motor of the mixer running at low speed, add the wet ingredients to the egg and flour mixture. Continue until all of the milk has been added. Scrape the sides of the bowl and mix again, ensuring there are no lumps.
Pour a layer of batter into each of the three prepared cake tins, and sprinkle a layer of cinnamon sugar over the surface of the batter. Divide the remaining batter between the three tins, pouring it over the cinnamon sugar. Use a toothpick to swirl the cinnamon sugar into the batter.
Bake the cakes in a preheated oven for 35 - 40 minutes or until a skewer inserted into the center of the cake comes out clean. Remove the cakes from the oven and allow the cakes to cool in their tins for 45 minutes. Remove the cakes from their tins and allow them to cool completely on a wire rack.
Dulce de Leche Frosting
In a bowl of an electric mixer fitted with a paddle attachment, add the butter and icing sugar. Beat at medium speed until well combined.
In a jug, combine the milk and vanilla. With the mixer running at medium-low speed, slowly add the milk mixture to the icing sugar mixture.
Once all the milk has been added, increase the speed of the mixture and beat for 5 minutes until the frosting is light and fluffy. Scraping down the sides of the bowl as you go.
Add the dulce de leche to the frosting and beat again until well combined.
To assemble the cake, trim off the domed tops using a sharp bread knife if necessary. Place one cake layer on a cake plate and top with a third of the frosting, smooth the frosting over the cake surface with a palette knife.
Continue with the remaining cake layers and frosting. Serve and enjoy!