Preheat the oven to 180°C. Brush each cavity of the mini sandwich tin with a thin layer of DIY Cake Release (see Note 2) and line the bases with a circle of greaseproof paper. Set aside.
In a large bowl, mix together the sugar and the orange zest, using the back of a spoon to press the ingredients into each other, until the zest is evenly distributed throughout the sugar.
Add the eggs to the sugar and zest mixture, then beat, using an electric whisk on high speed, until the mixture reaches the ribbon stage.
With the mixer running, add the oil to the mixture in a slow and steady stream, whisking until it is completely incorporated.
Add the orange juice and whisk again to combine.
Sift in the flour, baking powder and salt, then fold in using a large, metal spoon until no streaks of flour remain.
Divide the mixture evenly between the 12 cavities of the prepared tin, then bake in the preheated oven for 25-30 minutes, or until the cakes have turned a deep, golden colour and a sharp knife/skewer comes out clean (or with only a few moist crumbs).
Allow the cakes to cool in the tins for 5 minutes, then use your fingertips to gently pry each one away from the edges of the tin. Once the tin is cool enough to handle, remove the cakes from the tin, using the removable bases to push them out from the bottom, and transfer to a wire rack to cool completely before filling and decorating.