From: Art of Gluten-Free Baking (artofglutenfreebaking.com)
For the Cake
For the Streusel Topping
Grease and flour an 8 in x 8 in/20 cm x 20 cm pan with the extra butter and tapioca flour.
In a small bowl, mix together the milk and water. Add the 1 teaspoon of sugar and whisk until dissolved. Add the yeast and whisk until dissolved. Let proof while you prepare your other ingredients.
In a medium bowl, mix together the remaining sugar, flour, xanthan gum, baking powder, salt, and lemon zest if using.
In the bowl of a stand mixer fitted with the paddle attachment, place yeast mixture, eggs, cooled melted butter, and vanilla extract. Mix on low to combine. Add dry ingredients mixture. Beat on low to combine, and then beat on medium-high for 3 minutes.
Scrape into prepared pan. Dough will be very stiff and sticky. I put equal blobs into the four corners of the pan and then use a rubber spatula to smoosh it into all the nooks and crannies. Smooth top as best you can—it will still be uneven and bumpy. Don’t worry, this will be evened out with the rising and will be covered by the streusel.
Cover lightly with plastic wrap and place in a warm-ish, draft-free place to rise for about an hour. At the end of the rising time, the cake will have risen a bit and will be noticeably puffy.
Preheat oven to 350 degrees F/180 degrees C/Gas Mark 4. I let the oven preheat for a full 30 minutes to make sure it’s good and hot.
While you are preheating the oven. Make your streusel topping. Mix together all of the ingredients until all of the dry ingredients have bonded to the butter. I do the initial mixing with a spoon and then switch to using my hand to combine everything. Sprinkle the streusel clumps as evenly as possible over the top of the risen cake.
Bake at 350 degrees F for 45 minutes—until the cake has risen to about the top of the pan and a toothpick inserted in the middle comes out clean.
Cool in pan on a wire rack.
Store at room temperature, loosely covered. This is best the day it’s made, but it does retain its moistness for about 3 days.