Special equipment: a 12-cup Bundt pan
For the cake: Preheat the oven to 300 degrees F. Spray a 12-cup Bundt pan with nonstick spray and dust with flour.
In a stand mixer fitted with the paddle attachment, beat the butter and granulated sugar on medium-low until thick and fluffy, about 4 minutes. Add the eggs one at a time, scraping down the sides of the bowl between additions to ensure everything is homogenous. Slowly beat in the flour just until combined (the batter will be very thick).
Scrape the batter into the prepared Bundt pan. Bake until a toothpick inserted into the center of the cake comes out clean and the top is golden, 60 to 75 minutes. Cool in the pan on a rack for 20 minutes to allow the sides to release, then invert the cake onto the rack to cool completely.
For the glaze: Whisk the confectioners' sugar and cinnamon in a bowl. Add 2 tablespoons milk and whisk well. If the glaze drips off a spoon fluidly but slowly, it's done; if not, add more milk, a teaspoon or so at a time, until it does.
Drizzle the glaze over the top of the cake, letting it run down the sides. Serve.
The cake will keep at room temperature, covered with plastic, for up to 2 days.