Melt the butter using a double boiler or microwave.
Put almond flour, coconut flour, and erythritol in a large bowl and mix well.
Add the melted butter to the bowl and mix evenly.
Spread the dough evenly into a pan lined with parchment paper.
Bake in a preheated oven at 180°C for 15 minutes.
Let it cool completely, then cut into bars.