Add the room temperature butter, sugar, eggs, vanilla extract, and pumpkin to the bowl of a stand mixer or a large mixing bowl. Using the paddle attachment on the stand mixer, mix together ingredients on medium speed or you can use a handheld electric mixer on medium speed. Don't worry about all the lumps, they will work themselves out during the rest of the mixing process.
Add the spice blend and baking powder to the wet mixture and blend on medium speed until incorporated.
Begin to add in the flour in ½ cup increments and blend each addition on medium speed. Once you have 4 cups of flour into the wet batter, turn the mixture off and remove the bowl from the base.
Feel the dough and if it's still sticking to your fingers, add in 1-2 Tablespoon increments at a time until it no longer sticks to your fingers.
Form the Biscotti Logs
Flour the cutting board and turn the dough out onto the cutting board. Divide the dough in half and begin forming the logs. If it's hard to form the logs because the dough is sticky, you can add a little more flour.
Using half of the dough, form a log about 12" long by 3-4" wide. Flatten down so it looks like a rectangle. Transfer to the parchment lined tray. Repeat with the second half of the dough.
First Bake, Cool & Cut
Place the tray with the two logs into the preheated oven and bake for 30 minutes.
Transfer the logs to the cooling rack and let cool at least 30 minutes, but I usually let them cool 1-2 hours and sometimes overnight.
Make sure to preheat the oven to 350°F/175°C for the second bake.
Once they are cool to touch, slice them into ½"-¾" pieces. You can cut them straight or on the diagonal.
Second Bake
Place the cut biscotti with the flat area pointing up on the parchment lined tray. I use the same parchment as I used in the first bake.
Bake again to lightly toast the biscotti. The timing will vary based on how thick you cut the biscotti. You want them to have a slightly toasted look. For ½" thickness, bake 7-15 minutes. For ¾" bake 12-20 minutes.
Remove from oven and allow to cool on a cooling rack and cool completely before decorating.
Decorating with Cinnamon Spice Icing
Add the sifted powdered sugar to the bowl of stand mixer or large mixing bowl. Add the spices, butter, and corn syrup. Mix on low until well combined. Scrape down the sides of the bowl as needed. The mixture will look crumbly and dry.
Add in 2 Tablespoons of water and mix on low. Continue adding water, 1 Tablespoon at a time, until the icing is the consistency you want. It should be thin enough to coat the the cookies, but not so thin that it runs off the cookie. I usually find that ¼ cup of water is perfect.
Decorate the cooled biscotti cookies by either dipping them into the icing or simply spooning the icing over the tops like I did in the picture.