Cream butter, brown sugar and white sugar, add peanut butter and stir till combined.
Add egg, milk & vanilla, mix till smooth.
Combine flour, baking soda and salt in bowl. Add peanut mixture mixing well. Roll tablespoon sized amounts into round balls.
Arrange on cookie sheets.
Bake at 180 degrees celsius for around 6 minutes, take out of oven and flatten with a spatula and return to oven for further 6 minutes.
Remove from oven and turn over half the cookies and place a piece of chocolate on the overturned cookies, let sit for a minute till choc melts then place the marshmallow on top, press down with the other half of the cookies.