01 - Preheat the oven to 350°F and grease a standard donut pan.
02 - Whisk together flour, cocoa powder, sugar, baking powder, baking soda, and salt in a large bowl.
03 - In a separate bowl, whisk milk, espresso, eggs, vegetable oil, and vanilla extract until smooth.
04 - Pour wet ingredients into dry and mix gently until just incorporated without overmixing.
05 - Spoon or pipe the batter into the prepared pan, filling each cavity about two-thirds full.
06 - Bake for 10 to 12 minutes, or until a toothpick inserted comes out clean.
07 - Allow donuts to cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.
08 - Whisk powdered sugar, milk, peppermint extract, and cocoa powder if using, until smooth and pourable.
09 - Dip cooled donuts into the glaze and immediately sprinkle with crushed peppermint candies and mini chocolate chips if desired.
10 - Let the glaze set for 10 minutes before serving.