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1
Heat the oven to 350 degrees F and arrange a rack in the middle.
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2
Generously coat a 9-by-13-inch baking dish with butter and line the bottom with enough parchment paper to extend up the sides and over the edges of the two longer sides (this will help with unmolding).
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3
In a food processor, pulse the nuts, citrus zests, and half of the granulated sugar until finely ground.
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4
Add the rice flour, tapioca starch, baking powder, and salt and pulse to combine.
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5
In another bowl, mix together the orange and lemon juices.
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6
Measure out 3 tablespoons of the juice mixture and place in a separate bowl.
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7
In a stand mixer fitted with the paddle attachment, beat the butter and remaining granulated sugar on medium-high speed until light and fluffy.
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8
Add the eggs 1 at a time, mixing well after each addition.
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9
Add the larger measure of citrus juice (6 tablespoons).
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10
Gently stir into the nut mixture just until combined.
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11
Spread the batter in the prepared baking dish.
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12
Bake until its a light golden brown, about 30 minutes.
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13
Let cool in the dish for about 20 minutes, then turn out onto a wire rack set over a baking sheet to cool completely.
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14
Stir the powdered sugar into the remaining 3 tablespoons of citrus juice, then pour over the top of the cake and spread evenly.
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15
Let the glaze set for about 30 minutes before cutting into bars.
