Whisk together the butter, condensed milk, vanilla, salt and baking powder in a mixer until thoroughly combined - don’t worry if it looks lumpy! Add the flour and stir until a dough forms. Cover and refrigerate for 30 minutes, so it’s firm enough to handle but still tacky.
Preheat the oven to 180°C/356°F/Gas 4. Line a baking sheet with parchment paper. With damp hands roll level tablespoons of the dough into balls, then roll in the coconut to gently adhere. Transfer to the baking, spaced about 1 inch apart. Press each centre with your thumb to create a deep indentation. Bake for around 10 minutes until they are just set and golden on the bottom, but still soft - if needed carefully press down the centre indent with the end of a wooden spoon and allow to cool.
Melt the chocolate in a bowl over a pan of water and then carefully dip the base of the biscuits in the chocolate. Return to the lined baking sheet, bottom side up and allow to set.
When set turn the biscuits over so that the chocolate side is on the base. Bring the jam to the boil in a small saucepan and whilst still warm spoon about 3/4 teaspoon into the centre of each cookie. Pop in the fridge until set, then bring to room temperature.