For the topping: Preheat the oven to 325 degrees F.
Place the walnuts on a baking sheet in a single layer and toast until golden brown and fragrant, 5 to 7 minutes. Let cool slightly and chop. Put in a shallow dish with the bacon and mix together. Set aside.
For the apple cider glaze: In a small saucepot, add the apple cider, brown sugar, vanilla extract, cinnamon and a squeeze of lemon juice. Bring to a boil over high heat and then reduce to a simmer. Simmer until syrupy and reduced by half, about 10 minutes. Let cool.
Put the confectioners' sugar in a bowl and slowly pour in 1/4 cup of the glaze, a little at a time. Whisk together until smooth.
To top the doughnuts: Gently dip the top of each plain doughnut into the glaze and let any excess drip back into the bowl. Next, lightly dip the glazed tops into the chopped walnuts and bacon. Place on a wire rack to let set.