Prepare the pan. Line the bottom of a 20 cm (8 inch) ungreased round pan with parchment paper.
Mix dry ingredients. Whisk oat flour, cornstarch, baking powder, and salt. Set aside.
Make the yolk mixture. In another bowl, whisk egg yolks, Greek yoghurt, milk, oil, vanilla, and sweetener until smooth. Sift in dry ingredients and whisk until lump-free.
Whip the egg whites. In a clean, dry bowl, beat egg whites until foamy. Add cream of tartar or lemon juice, then gradually add sweetener. Beat to soft, glossy peaks.
Combine mixtures. Add one-third of the egg whites to the yolk batter to loosen it. Gently fold this mixture into the remaining whites until just combined. Don't overmix!
Bake. Pour into the pan, tap gently to release bubbles. Bake at 160°C (320°F) for 30–35 minutes, or until the top springs back and a skewer comes out clean with moist crumbs.
Cool upside-down. Immediately invert the pan onto a cooling rack. Let cool completely before releasing from the pan.
Make the frosting. Beat cold cream cheese and Greek yoghurt together for 20–30 seconds. Add powdered sweetener, beetroot or food colouring, rose essence, and vanilla. Beat on low speed until smooth and fluffy.
9. Assemble. Once the cake is completely cool, remove from the pan, frost with rose cream cheese, and chill for 30 minutes before serving.