Mix butter, egg, and 125 g brown sugar in a large bowl until fully combined. Stir in flour, baking powder, 50 g oatmeal, and 50 g desiccated coconut until fully combined. Cover the bowl and refrigerate for 2 hours.
10 minutes before baking, preheat the oven to 175°C (347°F). Line a baking sheet with baking paper.
In a small bowl, mix the remaining 25 g desiccated coconut with 25 g brown sugar.
Shape the dough into balls roughly the size of a walnut.
Brush each ball with condensed milk and press gently into the coconut-sugar mixture to coat. Arrange on the prepared baking sheet.
Bake on the middle rack for 15 minutes until the edges become light golden brown.
Cool on the baking sheet for 10 minutes, then transfer to an airtight container for storage.