First, prepare the cake. Preheat the oven to 375 degrees Fahrenheit. In a medium-sized bowl, mix together the flour, cinnamon, baking powder, baking soda, salt, nutmeg, allspice, and ginger until blended. Set aside.

Cream together the butter, granulated sugar, and brown sugar until smooth. Add the eggs, one at a time, mixing well after each addition. Add the olive oil, maple syrup, and pumpkin and mix until completely incorporated.

Mixing at low speed, gradually add the flour mixture to the pumpkin mixture, about 1 cup at a time, until it is just incorporated. Pour the batter into a roughly 8 x 13 inch lightly greased rectangular baking pan and bake for 45 minutes to 1 hour, or until a toothpick inserted into the center of the cake comes out clean. Allow to cool completely before glazing.

Next, prepare the toasted hazelnuts. Melt the butter and sugar in a small frying pan over medium heat, stirring to incorporate the sugar. Once the sugar has melted, add the nuts and stir to coat them in the mixture. Let them sit in the pan for a couple minutes until they start smelling slightly toasty, then stir it up again. Repeat this process until most of the nuts are lightly browned and toasted on part of their surface and the pan is very aromatic with the smell of toasted hazelnuts. Remove the pan from the heat and empty the mixture onto a baking sheet lined with parchment paper. Smooth it out so the nuts are in one layer, then sprinkle the salt over the nuts. Place the pan in the freezer for 10 minutes to get the nuts hard and crunchy, then remove them from the parchment paper and break the sheet of nuts apart with your hands. Set it aside.

To prepare the glaze, whisk together all the ingredients until smooth. Drizzle the glaze over the top of the cooled sheet cake and top with the toasted hazelnuts. Serve immediately.