Start by pouring the ale into a saucepan. To keep it from 'exciting' (foaming up), angle the saucepan and gently pour the ale down the side into the pan.
Stir in the 1 tsp of spices.
Gently heat until it comes to a boil, then lower the heat straight away and let simmer for a few minutes.
In these few minutes, whisk together the yolks and sugar.
Remove from the heat, let sit for a few minutes to bring the heat down, then add in the yolks and sugar to the ale.
Whisk until the sugar has dissolved and let simmer for 3-5 minutes.
Stir in the butter until fully mixed in.
With a hand blender, froth the ale until foam forms. Let sit to cool.
Using a spoon, hold back the froth as you pour the butterbeer into the beer stein. Leave about an inch of room on the top, spoon on the froth and serve.