Step 1: Preheat your oven to 425°F (218°C) and line a 12-cup muffin tin with paper liners or grease with butter.
Step 2: In a small bowl, prepare the streusel by mixing 1/2 cup brown sugar, 1/3 cup flour, and 1/2 teaspoon cinnamon. Cut in the cold cubed butter with a fork or pastry cutter until the mixture resembles coarse crumbs. Stir in 1/4 cup of the walnuts and set aside.
Step 3: In a large mixing bowl, whisk together the 2 cups flour, granulated sugar, baking powder, salt, and 1 1/2 teaspoons cinnamon.
Step 4: In another bowl, combine the melted butter, milk, egg, and vanilla extract until well blended.
Step 5: Pour the wet ingredients into the dry ingredients and stir gently with a spatula until just combined. Do not overmix; a few lumps are fine.
Step 6: Gently fold in the diced pears and the remaining 1/4 cup of toasted walnuts.
Step 7: Scoop the batter into the prepared muffin cups, filling them nearly to the top. Sprinkle the prepared streusel topping generously over each muffin.
Step 8: Place the tin in the oven and immediately reduce the temperature to 375°F (190°C). Bake for 22-25 minutes, or until a toothpick inserted into the center comes out clean.
Step 9: Allow the pear walnut streusel muffins to cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.