In a large heat-save mixing bowl, add egg whites. (Egg whites cannot contain any traces of yolks.)
Beat the egg whites until doubled in size, then add powdered sweetener (I use erythritol) and beat again to incorporate. Place the bowl over a pan of simmering water. Make sure the bowl is above the water line.
Using a hand whisk, whisk the whites and sweetener continuously until the sweetener dissolves, for about 5 minutes. The temperature of the mixture should reach 160 °F/ 70 °C to melt the sweetener.
Once the sweetener is melted, immediately transfer it to a stand mixer or beat it with an electric mixer.
Beat the mixture on medium-high for about 15 - 17 minutes until glossy.
If the bowl is still warm after mixing, let it reach room temperature, then add the butter one tbsp at a time. Incorporate butter fully before adding the next tbsp. ***See Notes for Troubleshooting
Once all the butter has been added, add the vanilla and mix until combined and fluffy, about 1 minute. Spread half over the peanut butter filling layer and add the other half to a pipping bag with your favorite decorating tip. Pipe your design and add a few crushed hazelnuts on top. Drizzle it with melted stevia-sweetened dark chocolate and refrigerate for 2 hrs. Enjoy!