Preheat the oven to 350 degrees F and grease a small bundt pan well.
In a mixing bowl, whisk together the flour, cocoa powder, sugar, baking soda, and salt.
In a second bowl, whisk the buttermilk, oil, egg, mint extract, and green food coloring until smooth.
Pour the wet mixture into the dry ingredients and stir until just combined with no dry streaks left.
Fold in 1/2 cup of the chocolate chips, then pour the batter into the prepared bundt pan.
Bake for 30 to 35 minutes until a toothpick comes out clean. Cool the cake in the pan for 10 minutes, then turn it out onto a wire rack to cool completely.
In a small saucepan, heat the heavy cream until it just starts to steam. Remove from heat, add the remaining 1/4 cup chocolate chips, and stir until melted and glossy.
Spoon the ganache over the cooled cake so it drips down the sides like a cauldron.