Preheat the oven to 180C or 160C Fan and line a 30x23x4cm traybake tin with parchment paper.
Mix the flour, baking soda, ginger, and mixed spice in a large bowl.
Gently heat the butter, treacle, syrup, and sugar in a saucepan until the butter has melted, then let it cool.
Pour the butter mixture into the dry ingredients, add the beaten egg, and mix with a wooden spoon.
Gradually add the milk and beat until smooth.
Pour the batter into the tin and bake for about 35 minutes until dark and springy to the touch.
Cool in the tin for 10 minutes, then transfer to a wire rack to cool completely.
