1. Preheat the oven to 350ºF (180ºC). Butter a 9-inch (23cm) square cake pan.
2. Make the topping by crumbling together the butter, sugar, flour and cinnamon with your hands or a pastry blender until the pieces of butter are broken up and are about the size of small peas. Set aside.
3. To make the batter, in the bowl of stand mixer fitted with the paddle attachment, or by hand in a bowl using a spatula or wooden spoon, cream the butter, sugar and lemon zest together until light and creamy, about 3 minutes. Add the eggs one at a time, stopping the mixer a few moments after you add each egg to scrape down the sides of the bowl.
4. Sift together the flour, baking powder, baking soda, salt, and cinnamon or nutmeg into a medium-sized bowl. With the mixer on low speed, add half the flour mixture, then stir in the buttermilk.
5. Add the remaining flour mixture, mixing just enough so it’s barely incorporated (there will still be dry bits of unincorporated flour), then remove the mixer bowl from the machine and using a flexible spatula to gently fold in the blueberries in, just until they are incorporated. Do not overmix – you don’t want to smash the blueberries and stain the batter.
6. Scrape the batter into the prepared baking pan and smooth the top. Strew the topping over the blueberry batter and bake until the buckle is lightly browned on top and feels just set in the center; it’ll spring back lightly when you touch the center. It’ll take about 55 minutes.
7. When the buckle is almost finished baking, make the syrup by heating the lemon juice and sugar in a small saucepan over medium heat, cooking it until it thickens. It’s done when the bubbles get larger, and when removed from the heat (which you might want to do a couple of times while it’s cooking, to check), the consistency will be like warm maple syrup. It’ll take about 5 minutes.
8. Remove the buckle from the oven and pour the warm lemon syrup over it, letting it soak in. Serve the buckle when it’s cool enough to slice. It’s good warm or at room temperature. Whipped cream, vanilla ice cream, or white chocolate-fresh ginger ice cream make nice accompaniments, although none are really necessary.