Preheat oven to 375º. Grease 12 standard muffin cups with butter or cooking spray, or line them with paper. Alternatively, place 12 silicone cupcake molds on a baking sheet. (This recipe always makes about 16 muffins when I use my silicone cups).

To make the topping, in a small bowl, stir together the sugar, flour and lemon zest. Stir in the melted butter until the mixture is crumbly. Add the pecans and stir to combine. Set aside.

To make the muffins, in a large bowl, stir together the flour, sugar, baking powder, baking soda, cinnamon, lemon zest and salt. Make a well in the center and add the egg, melted butter and buttermilk. Stir just until evenly moistened. The batter will be slightly lumpy. Sprinkle with the blackberries and gently fold in with a large rubber scraper just until evenly distributed, no more than a few strokes. Take care not to break up the berries. Do not overmix.

Spoon the batter into the prepared muffin cups, filling each to just below the rim of the cup. Top each muffin with the topping, dividing it evenly.

Bake until the muffins are golden, dry and springy to the touch and a toothpick inserted into the center comes out clean, 25 to 30 minutes. Transfer the pan to a wire rack and let cool for 10 minutes. Unmold the muffins. Serve warm or at room temperature.