Step 1

Adjust oven rack to middle position. Preheat oven to 350°F. Spray an 8-inch square baking pan with nonstick cooking spray.

Step 2

In large mixing bowl, whisk together rice flour, granulated sugar, tapioca starch, baking powder, and salt. In a measuring cup or a small bowl, whisk together buttermilk and lemon zest. Pour buttermilk mixture over dry ingredients. Whisk in vegetable oil and eggs until smooth.

Step 3

Pour batter into prepared cake pan. Bake until a cake tester inserted into center of cake comes out clean, about 30 minutes.

Step 4

Option 1: Simple Syrup Glaze Meanwhile, if making the Simple-Syrup Glaze, combine granulated sugar, lemon zest, and lemon juice in a small saucepan. Bring to a boil over medium-high heat, stirring gently until sugar dissolves. Reduce heat to low and cook syrup until it thickens slightly, about 5 minutes. Remove from heat and set aside.

Step 5

While cake is still warm, poke it all over with a fork. Brush top of cake with syrup and allow to absorb fully, about 5 minutes. Repeat until all syrup is absorbed.

Step 6

Option 2: Powdered Sugar Glaze If making Powdered Sugar Glaze, allow cake to cool completely. In small bowl, combine powdered sugar and lemon juice. Stir with a fork until smooth. Icing should be thick but spreadable; if the icing is too thick, add a little more lemon juice. Spread icing on cake and allow to set for 15 minutes before serving.

Step 7

Store cake, covered, on the counter for up to 3 days.