Preheat oven to 350F. Grease or line a 13x9 baking dish with parchment paper.
In a large mixing bowl, whisk eggs together to break up yolks (for best results, avoid beating). Add the sugar and oil, whisking well to combine. While stirring continuously, gradually sift in alternating milk and rice flour, whisking until incorporated and few clumps remain. Add in any mix-ins last.
Pour batter into prepared pan and bake for 50-55 minutes, until edges have browned and top has set, and a toothpick inserted into the center of pan comes out mostly clean. Any bubbles will flatten upon cooling.
Remove from oven and let cool for one hour before slicing. Store covered on counter for up to three days.