Position racks so that the cake will bake in the middle of an oven and preheat the oven to 350° F. Grease a 9-inch square pan; do not line or flour it. Set aside.

Place the flour, brown and granulated sugars, nutmeg, salt, and cloves together in a strainer or sifter and sift into a large bowl. Whisk to mix well. Using a spoon, form a well in the middle of the dry ingredients, then pour in the butter or oil and stir, breaking up clumps, until the mixture resembles coarse bread crumbs. Remove 3/4 cup of the mixture to a small bowl and reserve for later use.

Stir the baking powder into the mixture in the large bowl. Add the buttermilk and eggs and beat with an electric mixer at medium speed until smooth, about 1 minute.

Scrape the batter into the prepared pan and smooth the surface with a rubber spatula. Evenly distribute the reserved crumb mixture over the top, then scatter the nuts over it and sprinkle with the sugar-cinnamon mixture. Bake until the cake springs back when lightly touched in the center with your fingertip and a wooden skewer inserted into the center of the cake comes out clean, 45 to 50 minutes; if the top begins to brown too quickly during baking, cover loosely with aluminum foil.

Remove the pan to a wire rack to cool for at least 15 minutes. Cut into squares and serve warm or at room temperature directly from the pan.