Preheat the oven to 335°F (170°C). The slightly lower temperature prevents the streusel and cake top from over-browning before the inside is fully baked.
Grease an 8x8-inch square pan with butter, then line the flat bottom with an 8x8 piece of wax or parchment paper.
In a small bowl, combine the softened butter, white rice flour, shredded coconut, coconut sugar, maple syrup, cinnamon extract, nutmeg, and sea salt. Use a fork to mash the ingredients together until the mixture has a crumbly consistency. Set aside.
In a medium bowl, whisk together the oat flour, rice flour, cornstarch, monk fruit sweetener, baking powder, baking soda, and sea salt until evenly combined.
Crack the eggs into a large bowl and whisk to combine. Add the carton coconut milk, avocado oil, apple cider vinegar, and vanilla extract. Whisk thoroughly to combine.
Pour the wet mixture into the dry mixture. Using a spatula, fold the ingredients together until just combined. Use a whisk if needed, but be careful not to over-mix the batter.
Pour half of the cake batter into the prepared pan. Sprinkle 1/3 of the streusel evenly over the batter. Pour the remaining cake batter on top, then cover the top layer with the remaining 2/3 of streusel.
Bake at 335°F for 25–30 minutes. Test the center with a toothpick — it should come out clean (or with a few moist crumbs) when the cake is done.
Cool the cake on a wire rack for 20–30 minutes before slicing. Serve and enjoy!