Preheat oven to 160 degrees celsius fan forced (180 degrees if conventional).
Grease and line a rectangular slice tray (approx 16cm x 26cm) with baking paper - allowing it to hang over the edges.
To make the base, sift the plain flour into a bowl.
Add the brown sugar and coconut and stir to combine.
Add the melted butter and mix well until crumbly and completely combined.
Press the mixture firmly into the base of the prepared tin and smooth with a spoon.
Bake for 15 minutes or until lightly golden.
While the slice is still warm, spread the jam over the top.
Cut each marshmallow in half (using scissors is easiest).
Place the marshmallow halves (cut-side down) onto the jam.
Place back into the oven for 2 minutes.
Use a spoon or spatula to gently press the marshmallows down so that you have a flat surface.
Leave in the tin to cool completely so that the marshmallows can start to firm up again.
Place the dark chocolate melts and butter into a microwave-safe bowl. Heat in 30 second bursts (50% power) until the chocolate has just melted - stir at each interval.
Very gently spread the chocolate over the marshmallows.
Place into the fridge to set completely (approx 3 hours).
Allow the slice to come to room temperature before cutting into slices.