Step 1In a food processor, pulse together the sugar, flour, salt and butter until the pieces are the size of coarse crumbs.
Step 1Heat the oven to 350 degrees. Butter a 9-inch glass pie plate and set aside.
Step 2In a large bowl, sift together the flour, cornmeal, baking powder and salt.
Step 3In another large bowl, beat the 1/4 cup of butter with the sugar and egg until it is fairly light and fluffy. Add half of the milk and beat until smooth. Gradually add the remaining milk while beating.
Step 4Stir the dry ingredients into the wet ingredients just until well moistened. It will be the texture of cake batter. Fold in the plum pieces. Pour the batter into the pie plate and spread evenly. Scatter the topping mixture evenly over the top. Bake until the top is golden brown and a toothpick inserted in the center comes out clean, about 45 minutes.
Step 5Remove from the oven and let stand 20 minutes before serving. (If you serve the buckle the next day, warm it briefly in the microwave before serving.)