Cornmeal Blueberry Cake (grain-free)

We have two blueberry plants in our backyard and they are producing a good number of blueberries (for some reason the birds don't seem interested this year.) I went looking for recipes that use berries and cake across Martha Stewart's Cornmeal-Berry Sheet Cake. I decided that since it's just in a 9x13 pan (not really a sheet pan) I could probably adapt it to be grain-free and slightly lower-sugar. So I did. Follow the links to the original recipe; my recipe is below. I baked this on July 4th but in reality we ate it for breakfast on July 5th.

Cornmeal Blueberry Cake
(a riff on Martha's Cornmeal-Berry Sheet Cake) 

Ingredients

1 1/2 sticks unsalted butter, room temperature, plus more for pan
1 1/2 cups almond flour
3/4 cup fine yellow cornmeal
1/4 cup tapioca flour (could also use arrowroot or leave out)
1 1/2 teaspoons baking powder
3/4 teaspoon coarse salt
1 1/2 cups coconut sugar
3 large eggs
1 1/2 teaspoons pure vanilla extract
3/4 cup buttermilk*
1 1/2-2 cups blueberries

Directions

Preheat oven to 325 degrees. Butter a 9-by-13-inch baking pan and line with parchment, leaving overhang on long sides. Butter parchment.

Whisk together flours, cornmeal, baking powder, and salt in a bowl.

Beat butter and sugar on medium-high speed until well combined, about 2 minutes. Add eggs, one at a time, beating well and scraping down sides of bowl as needed. Beat in vanilla. Add flour mixture in 3 batches, alternating with 2 batches of buttermilk and beating until just combined. Spread batter in prepared pan. (If using my adapted version here, batter is rather loose and soft, do not despair!)

Sprinkle berries over top or gently incorporate into batter. Bake until golden brown and a toothpick inserted into center comes out clean, about 1 hour. Let cool completely on a wire rack. Remove cake with parchment and cut into pieces (3 or 4 on short side, then 4 or 6 on long side).