Preheat the oven to 160°C (325°F) and line a 20cm (8-inch) square baking tin with baking parchment.
Gently melt the Unsalted Butter with the Golden Syrup, Black Treacle and Soft Brown Sugar in a medium saucepan over low heat, stirring until the sugar has mostly dissolved and the mixture is smooth — do not boil.
Remove the pan from the heat and stir in the Oatmeal so it soaks up the warm syrup mixture.
Sift together the Self-raising Flour, Ground Ginger, Mixed Spice and Bicarbonate of Soda into a large bowl, then pour in the syrupy oat mixture and fold to combine.
Beat the Egg lightly with the Milk in a small jug, then add to the batter and mix until you have a thick, sticky dough — add a splash more milk if it seems too dry but keep it dense.
Spoon the mixture into the prepared tin and press evenly into the corners with the back of a spoon or an offset spatula.
Bake for 25–35 minutes until the top is firm and a skewer inserted into the center comes out with a few moist crumbs (the cake should be slightly sticky but cooked through).
Allow the parkin to cool in the tin for at least 15 minutes, then lift out on the parchment to cool completely on a wire rack. For best flavor and texture, wrap and leave overnight to mature before slicing.
Slice into squares and serve thinly — it’s excellent on its own or with a smear of butter.
