Melt you butter and allow to cool for a few minutes
In a mason Jar, add egg yolks, mustard, salt, lemon juice, apple cider vinegar and quickly mix with an immersion blender (you can also use a blender)
With immersion blender running, add a little butter to the mixture and blend until it thickens.
Continue adding butter a little at a time and blend. Make sure you don't add too much butter at once or the mayo will break
As the mayo begins to thicken, you begin to slowly drizzle remaining butter into the mixture
Once all the butter is added, continue blending for a few seconds to thicken up the mayo. The longer you blend it, the thicker it will be but it will harden more in the fridge
For a creamier mayo and one that doesn't solidify as much in the fridge, add 1/4 tsp of tara gum
Store in an air tight container for up to 10 days