Preheat the oven to 180°C and prepare three 20cm round cake tins by brushing them with sunflower oil and lining them with baking paper. In the bowl of an electric mixer fitted with a whisk attachment, combine the eggs, granulated sugar, and oil. Beat on medium speed until well blended, about 2 minutes. Sift together the salt and flour, then add the dry ingredients to the egg mixture. Mix on medium speed just until combined. In a jug, mix the milk and vanilla extract. With the mixer running on low speed, slowly pour the milk mixture into the batter. Scrape down the sides of the bowl and mix again to ensure a smooth, lump-free batter. Divide the batter evenly among the prepared tins, then sprinkle cinnamon sugar over the surface of each layer. Pour the remaining batter evenly over the cinnamon sugar, and use a toothpick to gently swirl the cinnamon sugar into the batter. Bake for 35 to 40 minutes, or until a skewer inserted in the center comes out clean. Allow the cakes to cool in their tins for 45 minutes before removing them and placing them on a wire rack to cool completely.

In the bowl of an electric mixer fitted with a paddle attachment, beat the butter and icing sugar on medium speed until combined. In a jug, mix the milk and vanilla extract. With the mixer on medium-low, gradually add the milk mixture to the frosting and continue beating. Once fully incorporated, increase the speed and beat for 5 minutes until the frosting becomes light and fluffy, scraping down the sides of the bowl as needed. Add the dulce de leche and mix again until smooth and fully combined.

If needed, level the domed tops of the cake layers using a sharp bread knife. Place the first cake layer on a serving plate and spread one-third of the frosting evenly over the top using a palette knife. Repeat with the remaining layers and frosting. Serve and enjoy!