Preheat the oven to 180C (160C fan-assisted / 350F).
Put the sugar and butter in a bowl and beat together until very pale and fluffy.
Whisk in the eggs, Biscoff spread, milk and vanilla.
Fold in the flour and salt until combined into a thick batter.
Spoon into the tin and gently level off.
Bake in the middle of the oven for 45 minutes, or until it's firm in the middle.
Cool in the tin on a wire rack.
Make the biscoff frosting
Put the butter, icing sugar, salt, Biscoff spread and milk in a large bowl and whisk until pale and creamy.
Decorate your Biscoff traybake
For the frosting, you can either spread it onto the cake with a palette knife, or you can transfer it to a piping bag fitted with an open star tip and pipe rosettes all over the cake.
Crumble half of the Biscoff biscuits and scatter over the top.
Leave 3 or 4 Biscoff caramelised biscuits whole, break the rest into pieces and place on top of the cake.
Allow to set somewhere cool for 15 minutes before cutting into generous squares to serve.