Preheat oven to 400ºF. Grease a mini muffin tin or similar small mold with softened butter. Set aside.
In a small pan over medium heat, add butter and melt. Once melted whisk butter continuously until the butter has browned. Keep an eye on it as this can happen quick!
Once the butter is brown, immediately transfer to a small bowl. Set aside to cool slightly.
In a medium bowl, add sugar and all citrus zests. Massage sugar and zest together to combine and release oils from zest into the sugar.
Add in almond flour, all purpose flour, and salt. Whisk until combined and there are no more almond flour clumps.
Add in egg whites and vanilla, and mix until just moistened. Stir in brown butter until just combined. Don’t over mix!
Fill the molds up about 3/4 of the way up. Top with pistachios.
Bake for 11-14 minutes until puffed up and edges are golden. Let cool in the pan for 5 minutes, then transfer to a wire rack. Serve warm or room temperature. These are great with coffee or tea!
Store extras in an airtight container on the counter. They taste great the next day when stored this way :)