Preheat oven to 350ºF and line the muffin tins with paper cupcake liners.
In a medium bowl, sift together the coconut flour, baking powder and matcha powder. Set aside.
In a separate bowl, whisk together the coconut oil, sugar, eggs and vanilla bean paste, until combined.
Add the flour mixture to the egg mixture and fold until fully incorporated.
Scoop into the cupcake liners using a small ice-cream scoop for the minis and a regular size scoop for the larger cupcakes, filling each about half way.
Bake the mini cupcakes for 15-20 minutes and the larger for 25-30 minutes. This may differ based on your oven, so keep an eye on them as they bake.
Insert a cake tester (or toothpick) into the center to test for readiness. When it comes out clean, they are ready!
Remove from oven and allow to cool in pan on a cooling rack for about 10 minutes before transferring them directly onto the cooling rack.
Allow to cool completely before decorating.
To make buttercream:
Using a clean towel and some lemon juice or vinegar, clean your mixing bowl, whisk attachment and any other utensils you will use, to remove any traces of grease.
Fill a heavy duty pot about a quarter of the way with water and place on the stove over medium heat to simmer.
Add egg whites, sugar and salt to the mixing bowl and whisk together lightly with a hand whisk, just until combined. Fit the bowl onto the pot, ensuring the bottom of the bowl is not in contact with the water in the pot. Insert a thermometer. Whisk the egg white mixture constantly, but gently, until temperature reaches 160°F.
Once at the correct temperature, remove carefully from heat, wipe the moisture away from the bottom of the bowl and transfer mixing bowl to the stand mixer. With the whisk attachment, begin to whip the meringue until it is thick, glossy, and the bottom of the bowl is no longer warm to the touch.
Do not begin adding butter while the bowl is still warm.
Switch from your whisk to paddle attachment and with mixer on low speed, add butter cubes, a little at a time, until incorporated. Continue to mix until it has reached a silky smooth texture.
Add vanilla beans and passion fruit compound, continuing to beat on low speed until well combined.