Center a rack in the oven; preheat oven to 350°; generously butter a 6-mold mini Bundt pan.
Make the swirl: toss the nuts, cocoa, and sugar together in a bowl; set aside.
Make the cake: in a bowl, whisk the flour, baking powder, and salt together; set aside.
Working with a stand mixer fitted with paddle attachment, or with a hand mixer in a large bowl, beat the butter and sugar together on medium speed until smooth, about 3 minutes (always scrape down sides of bowl as needed when mixing).
Add in the egg; beat 1 minute; then beat in the vanilla (don’t be concerned if the mixture looks curdled—it will smooth out soon).
Decrease mixer speed to low; add in half the flour mixture, mixing only until incorporated.
Add in the milk, and when it is blended into the batter, add the remaining flour mixture, again mixing just to incorporate.
Add in the melted chocolate; mix to blend.
Fill each of the mini Bundt molds with a little batter, then divide the swirl mixture evenly among the molds; top off the Bundts with the remaining batter.
Bake 20-22 minutes or until test done (pick comes out clean).
Transfer the pan to a rack; let rest for 5 minutes; then invert them onto the rack and let them cool to room temperature.
Make the glaze: melt the chocolate in a heatproof bowl over a saucepan of simmering water OR in a microwave oven (my preferred method).
Stir in the corn syrup; using a small offset metal spatula or a table knife, spread the shiny glaze over the tops of the cooled Bundts, then scatter the nuts, if using, over the glaze.
Let the glaze set at room temperature; it will take about 15 minutes.