Preheat oven to 350°F. Lightly butter an 8-inch square baking pan and line bottom with wax paper, then lightly butter paper.
Beat together butter, sugar, and zest in a large bowl with an electric mixer until creamy, about 2 minutes. Add eggs 1 at a time, beating until just incorporated. Whisk in sour cream and orange juice, then add remaining ingredients (except confectioners sugar), whisking until just combined.
Transfer batter to baking pan and bake in middle of oven until golden and a wooden pick inserted in center comes out clean, about 30 to 40 minutes.
Cool cake in pan on a rack 10 minutes, then turn out onto rack. Discard paper and cool. Dust with confectioners sugar.