Mix the flour, oatmeal, baking powder and salt. Soften the butter a little before adding to the dry mix and rubbing it in until it looks like breadcrumbs.
Add the sugar and milk/water and mix together until you have a ball of dough. If it won't come together properly, add a little more milk/water a teaspoon at a time.
Sprinkle the work surface with a little flour and roll out the dough until it is about 3mm thick. Cut out discs of dough with a cookie cutter, place on a greased baking sheet and bake for 20-25 minutes until they're a light golden brown.
When they are ready, leave to cool on a cooling rack whilst you melt the chocolate in a bain marie.
Top each cookie with chocolate and leave to cool again.