Preheat oven to 350F. Butter a 12-cup muffin tin, or line with muffin liners.
For the streusel topping, add the flour, brown sugar, and butter to a bowl and mix with your hands until it clumps together. Mix in the hazelnuts and set aside.
Whisk together the flour, brown sugar, cinnamon, ginger, nutmeg, allspice, baking powder, baking soda and salt until fully mixed.
Add the eggs, vegetable oil, buttermilk, and vanilla extract to a separate bowl and whisk together until the mixture is smooth. Then add the dry ingredients to the wet ingredients and mix until the batter comes together and there are no dry bits of flour left.
Add the chopped pears and carefully fold them into the batter.
Divide the batter into the muffin tins. You want to fill each muffin cup almost to the top with batter, leaving a little room for the streusel topping. The batter should make between 10-12 muffins, but you can definitely make more like 12-14 smaller muffins by just filling the tins a little less. Top with the streusel topping.
Bake the muffins for 20-25 minutes until risen and golden brown. Allow to cool for 10 minutes or so before taking out of the muffin tin.