Step 1: Preheat your oven to 400°F (200°C) and line a standard 12-cup muffin tin with paper liners or grease well with butter.
Step 2: In a small bowl, prepare the streusel by mixing 1/2 cup flour, 1/2 cup brown sugar, and 1/2 tsp cinnamon. Cut in 4 tbsp of cold cubed butter until crumbly, then fold in 1/2 cup of chopped pecans. Set aside in the fridge.
Step 3: In a large bowl, whisk together the 2 cups flour, granulated sugar, baking powder, salt, and cinnamon.
Step 4: In a separate medium bowl, whisk the melted butter, eggs, sour cream, milk, maple syrup, and vanilla extract until smooth.
Step 5: Gently fold the wet ingredients into the dry ingredients using a spatula. Mix until just combined; do not overmix. Fold in the remaining 1/2 cup of chopped pecans.
Step 6: Divide the batter evenly among the muffin cups. Top each muffin generously with the chilled pecan streusel mixture.
Step 7: Bake at 400°F for 5 minutes, then reduce the heat to 350°F (175°C) and bake for another 13-15 minutes, or until a toothpick comes out clean.
Step 8: Let the muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely.