Step 1: In a shallow frying pan, heat 200 grams of the butter, with the mace and one tablespoon of tarragon leaves over a medium high heat, until it is nut brown. Remove from the heat and allow butter to cool for about 15 minutes, until the solids separate and the flavours are infused.
Step 2: Shred the prawn meat using two forks or chop by hand. Place into a mixing bowl, then mix in the salt and add pepper to taste.
Step 3: Add the Verjuice, lemon zest and a teaspoon of fresh tarragon and mix in well.
Step 4: Add the rest of the butter to a clean pot and heat gently to clarify; you may have to skim off the foam. Set aside.
Step 5: Transfer prawn meat to a larger bowl, and then pour in the flavoured clarified butter, making sure to leave the solids behind. Stir through and check seasoning.
Step 6: Pack the prawn meat into four small ramekins or one large mould, smoothing the surface with the back of a spoon.
Step 7: Pour the remaining clarified butter over the top of each ramekin or the mould, forming a thin film.
Step 8: Refrigerate until butter is set, and then remove from the refrigerator 10 minutes before serving.
Step 9: Preheat a fan forced oven to 220C.
Step 10: Brush slices of bread with the remaining butter and bake in the preheated oven until crisp.
Step 11: Serve the potted prawn with toast, some rocket leaves and a wedge of lemon.