Method
Melt the butter in a pan over a low heat. Allow it to gently bubble, but not burn. As soon as you see the buttermilk browning, remove the butter from the heat and let it settle. Carefully pour the hot butter through a fine sieve into a jug or bowl, making sure you leave the buttermilk behind.
Clean the pan and pour in two-thirds of the melted butter. Add the lemon zest, mace, cayenne and thyme, along with a good pinch of salt and a generous twist of black pepper. Allow the butter gently to infuse over a low heat for 3–4 minutes, then remove the pan from the heat and add the shrimps, stirring them slowly into the butter.
After removing the thyme sprig, divide the shrimps and butter equally between four ramekins or pots and place in the fridge to set. When the mixture is firm, pour over the last of the melted butter to seal. Return to the fridge.
Remove the potted shrimps from the fridge 30 minutes or so before you intend to eat them. Serve on warm toast with a wedge of lemon for squeezing.