To make cake: Preheat oven to 350 F. Lightly butter bundt pan then dust with cocoa powder. Turn pan, tapping, to coat on all sides; tap out excess.
In small saucepan over low heat, melt 1 cup butter, 1/2 cup cocoa powder and water, whisking until smooth; let cool.
Transfer to bowl of mixer and lightly beat in sugar, peppermint extract and eggs.
In medium bowl, mix flour, baking powder, salt and baking soda. Add about 1/3 of dry ingredients to chocolate mixture and beat on low speed to combine. Add 1/4 cup buttermilk and beat again. Repeat steps ending with flour mixture, mixing only until combined. Fold in walnuts.
Spread in prepared pan and bake 50-60 minutes, or until toothpick inserted in center comes out clean. Let cool then invert onto serving plate or platter.
To make Chocolate Peppermint Truffle Glaze: While cake bakes, melt chocolate chips, butter, milk and peppermint extract together until smooth. Let cool slightly to thicken then drizzle over cake. Top with walnuts and peppermint candies.