Preheat our oven to 350 degrees and line a 9 inch cake pan with parchment paper, rubbing the sides with coconut oil.
In a medium bowl, stir together the oat flour, baking powder, cinnamon, baking soda and salt. Set aside.
In a large bowl, using an electric hand mixer on high speed, beat together the egg, egg yolk, coconut sugar, Greek yogurt, coconut oil, extract and orange zest until well combined, about 30 seconds.
Pour in the champagne and whisk until well combined.
Pour the dry ingredients into the wet ingredients and stir until smooth and well combined. Pour into the prepared cake pan and smooth out evenly.
In a medium bowl, stir together the oats and coconut sugar for the crumble. Add in the melted coconut oil and use your hands to mix until the oil is evenly distributed, and you have a crumbly mixture.
Sprinkle the crumbs evenly over the top of the cake in a thick layer, lightly pressing them into the batter.
Bake until the crumbe are golden brown, the sides of the cake rise and a toothpick inserted in the center comes out clean, about 38-40 minutes.
Let cool in the pan for 30 minutes. Then, gently transfer to a wire rack to cool COMPLETELY before slicing. **
Slice and DEVOUR (preferably with a Mimosa!)