Preheat the oven to 375 degrees F (190 degrees C). Grease a 12-cup muffin tin.
Mix flour, sugar, baking powder, cinnamon, and salt together in a large bowl.
Combine milk, vegetable oil, and egg in a separate bowl; beat with an electric mixer until thoroughly blended. Pour into flour mixture and mix until moist; do not overmix. Fold in pecans and transfer about 2/3 cup batter to each prepared muffin cup.
Bake muffins in the preheated oven for 8 minutes.
While muffins are baking, mix pecans, sugar, melted butter, and cinnamon for the topping in a small bowl. Remove muffins from the oven, top with pecan-butter mixture, and return to the oven. Bake until muffins are golden and tops spring back when lightly pressed, 7 to 12 minutes.