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1
Move oven rack to lowest position.
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2
Heat oven to 375F (190C).
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3
Sift together powdered sugar, flour, and cocoa.
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4
Beat egg whites and cream of tartar in large bowl on medium speed until foamy.
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5
Beat in granulated sugar, 2 tablespoon at a time, on high speed, adding salt with the last addition of sugar.
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6
Continue beating until stiff and glossy.
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7
Do not underbeat.
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8
Sprinkle cocoa mixture, 1 tablespoon at a time, over meringue, folding in just until cocoa mixture disappears.
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9
Spread batter in ungreased tube pan, 10x4 inches.
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10
Gently cut through batter with metal spatula.
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11
Bake 30 to 35 minutes or until cracks feel dry and top springs back when touched lightly.
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12
Invert pan onto metal funnel or glass bottle about 2 hours or until cake is completely cool.
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13
Remove from pan.
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14
Slice off top of cake about 1 inch down; set aside.
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15
Cut down into cake 1 inch from outer edge and 1 inch from edge of hole, leaving substantial inchwalls inch on each side.
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16
Remove cake within cuts with curved knife or spoon, being careful to leave a base of cake 1 inch thick.
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17
Spoon sherbet into cake cavity; smooth top.
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18
Replace top of cake.
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19
Cover and freeze about 3 hours or until firm.
