This tender sour cream-pumpkin Bundt cake features a crunchy brown sugar streusel on the bottom and a spiced pumpkin glaze on top for a cozy dessert.
Preheat the oven to 350 degrees F (175 degrees C). Grease and flour a 12-cup fluted tube pan (such as Bundt).
Combine brown sugar, 2 teaspoons cinnamon, and allspice in a bowl; cut in 4 teaspoons butter until crumbly. Set streusel aside.
Combine flour, 1 tablespoon cinnamon, baking soda, and 1 teaspoon salt in a bowl. Set aside.
Beat white sugar and 1 cup butter together in a large bowl with an electric mixer until light and fluffy. Slowly beat in eggs while continuing to beat. Stir in 1 cup pumpkin purée, sour cream, and 2 teaspoons vanilla extract; gradually beat in flour mixture until incorporated.
Spoon ½ batter into the prepared pan, sprinkle streusel over batter, avoiding sides of the pan, top with remaining ½ batter.
Bake in the preheated oven until a knife inserted into center comes out clean, 55 to 60 minutes. Cool in the pan for 30 minutes, then invert cake onto a wire rack to finish cooling.
Meanwhile, combine confectioners' sugar, ½ teaspoon cinnamon, and ⅛ teaspoon salt in a bowl. Combine 2 tablespoons pumpkin purée, milk, 1 teaspoon melted butter, and ¼ teaspoon vanilla extract in a bowl until smooth; whisk into confectioners' sugar mixture until well combined. Add more milk until glaze reaches desired consistency.
Drizzle glaze over cooled cake.