In a large microwave-safe mug, stir together the flour, cocoa powder, coconut sugar, baking powder, and salt until no clumps remain.
Add the milk, Greek yogurt, and vanilla. Stir until the batter is completely smooth, scraping the bottom corners of the mug.
Fold in 1 tbsp chocolate chips.
Microwave 60 to 75 seconds, checking at 60. The top should look set with a slightly soft center. Do not overcook.
For the ganache, microwave 1 tbsp chocolate chips with 1 tsp milk in short bursts, stirring until glossy and pourable. Pour over the warm cake.
Top with candy pieces while the ganache is still soft. Serve warm.