Step 1

Position a rack in the center of the oven and preheat to 350ºF.

Step 2

Line an 8-inch square pan with parchment paper on all sides.

Step 3

In a large, deep bowl, rub the lemon zest into the sugar until the sugar feels damp.

Step 4

Whisk in the eggs to combine, then whisk in the oil, sour cream, and lemon juice until smooth.

Step 5

Place a strainer over the bowl and sift in the sweet rice, millet, oat, and tapioca flours with the baking powder, baking soda, and salt.

Step 6

Whisk until combined, then whisk in the poppy seeds.

Step 7

Pour the batter into the prepared pan and spread smooth.

Step 8

Bake until a toothpick inserted in the center comes out clean or with a few moist crumbs, 30-35 minutes.

Step 9

Let the cake cool to room temperature, 1-2 hours. Turn the cake out, peel away the parchment, and place the cake upright on a serving platter or board.

Step 10

Spread the cake with the frosting, decorate the top with poppy seeds, lemon zest, or lemon slices, and serve!