Preheat oven to 350F. In a medium bowl, whisk together the flour, graham crumbs, cocoa, baking powder and salt.
In a large bowl, beat the butter and sugar for 3-4 minutes, until fluffy. Beat in the egg and the vanilla. On low speed or using a spatula, stir in the dry ingredients, along with the coconut and walnuts.
Roll dough into 1 1/2-inch balls and place a couple inches apart on a parchment-lined baking sheet. Flatten the balls slightly and use your thumb or the handle of a wooden spoon to create an indentation about half an inch in diameter.
Bake for 14 minutes, until just set. Remove and use the back of a small spoon to gently reform the indentations. Transfer to a rack to cool completely.
To make the filling, beat the butter, icing sugar, custard powder, cream, vanilla and salt until smooth and fluffy, adding a bit more cream or icing sugar as needed to create a spreadable frosting. Place the filling in a piping bag fitted with a 1/2-inch plain tip, or in a zip-lock bag; seal and and cut off one corner.
Pipe some frosting into each cooled cookie. In a small bowl, melt the chocolate chips and butter in the microwave in 10 second increments, stirring in between, until smooth. Dip the cookies halfway into the chocolate to coat, or drizzle them with a fork. Set back on the cooled baking sheets and let them set. Makes about 2 dozen cookies.