Preheat your oven to 375°F. Prepare a 12-cup muffin tin by greasing it or lining with paper liners. Prepare the streusel topping by mixing the 1/4 cup flour and 2 tablespoons sugar in a small bowl, then add the cold cubed butter. Work the butter into the flour mixture with your fingertips, rubbing until it resembles coarse breadcrumbs with some pea-sized pieces still visible. Set this streusel aside and keep the sparkling sugar nearby.

Take 1 to 2 tablespoons from the 2 cups of flour and toss it with the black raspberries to coat them lightly—this coating prevents them from sinking during baking. In a large mixing bowl, whisk the room-temperature eggs, oil, sugar, buttermilk, vanilla, and finely grated lemon zest together until everything is well blended and slightly emulsified.

In a separate small bowl, whisk together the remaining flour (after using some to coat the berries), baking powder, baking soda, and salt to distribute the leavening agents evenly. Pour this dry mixture into the wet ingredients and use a spatula to fold gently until just combined—the batter should remain slightly lumpy. Avoid overmixing, as this develops gluten and creates tough, dense muffins.

Gently fold the flour-coated raspberries into the batter as the very last step, being careful not to crush or break them. Portion the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. For consistency, use a 1/4-cup measure or ice cream scoop to divide the batter.

Top each muffin with about 1 teaspoon of the streusel mixture, then add a small pinch of sparkling sugar to each. Transfer the muffin tin to the preheated oven and bake for 25 to 27 minutes, until the muffin tops turn golden brown and a toothpick inserted into the center of a muffin comes out with a few moist crumbs clinging to it—not completely dry.

Let the muffins cool in the pan for 10 minutes to allow them to set and firm up enough to remove cleanly. After 10 minutes, turn them out onto a wire rack to cool completely.