Preheat the oven to 350°F plus butter and flour a Bundt pan thoroughly.
In a medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, allspice, and salt.
In another bowl, whisk together the pumpkin purée, buttermilk, and vanilla extract.
In a large bowl, beat the butter and granulated sugar with an electric mixer on medium-high speed until pale and fluffy, about 3 to 5 minutes.
Add the eggs and beat for 1 more minute.
Reduce mixer speed to low to add the dry flour mixture and pumpkin mixture alternately in batches, beginning and ending with the flour mixture. Mix until the batter is just smooth.
Spoon the batter into the prepared Bundt pan andmake sure to tap the pan gently on the counter to eliminate air pockets and even out the batter.
Bake for 45 to 50 minutes, or until a skewer inserted into the center comes out clean.
Cool the cake in the pan on a wire rack for 15 minutes and then place a wire rack over the pan, invert the cake onto it, and cool for 10 minutes more.
To make the glaze, melt the butter in a small saucepan. Stir in the sifted powdered sugar.
Add cream or milk one tablespoon at a time, stirring until the glaze reaches your desired consistency.
Drizzle the glaze over the cooled cake while it’s still on the wire rack with a tray or plate underneath to catch the excess.