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, cinnamon, and allspice in a bowl. Cut in butter until streusel is crumbly.

Combine flour, cinnamon, baking soda, and salt in a medium bowl.

Cream together sugar and butter using an electric mixer in a separate large bowl. Slowly add eggs while continuing to beat. Add pumpkin, sour cream, and vanilla extract and mix well. Gradually add flour mixture and beat into batter.

Spoon 1/2 of the batter into the prepared pan. Sprinkle streusel over batter, avoiding sides of the pan. Top with remaining batter.

Bake in the preheated oven until a knife inserted in the center comes out clean, 55 to 60 minutes. Let cake cool for 30 minutes in the pan; invert onto a wire rack to finish cooling.

While cake continues to cool, combine confectioners' sugar, cinnamon, and salt in a medium bowl. Combine pumpkin, milk, melted butter, and vanilla extract in a separate bowl. Add pumpkin mixture to sugar mixture using a wire whisk. Add more milk until glaze reaches desired consistency, being careful to not make it too thin, as a thick glaze will drizzle nicely also.

Drizzle glaze over cooled cake.