Grease an 8x8 inch baking pan and line it with parchment paper, leaving a few inches on each side for easy removal.
In a large bowl, cream the ½ cup unsalted butter and 1 cup granulated sugar until light and fluffy, about 4 minutes, scraping down the sides of the bowl as needed.
Beat in the 1 large egg and 1 teaspoon pure vanilla extract, then beat in the 2 teaspoons baking powder and 1 teaspoon fine sea salt.
On low speed, mix in half of the 2 ¼ cups gluten free flour, then beat in the ½ cup whole milk, and finish with the flour until incorporated. The batter will be thick.
Reserve about ¼ cup of the 2 cups whole cranberries and, using a rubber spatula, fold in the remaining cranberries.
Spread the batter evenly into the prepared pan and scatter the reserved cranberries over the top of the cake.
Preheat the oven to 350℉, and rest the batter while the oven heats. Then, place the cake in the oven and bake for 50-60 minutes, checking at 50 minutes, until set and a tester inserted in the center comes out clean.
Set the pan on a wire rack and cool for 15-20 minutes in the pan. Remove the cake from the pan using the extra parchment, and let cool to room temperature before serving.
Notes
The batterwill be thick. Note that the batter will be quite thick, especially after adding the cranberries. Don't worry, this is normal! Use a rubber spatula to spread and smooth it into the pan.
Try different-sized pans. I like a thicker cake, so I baked this in an 8x8-inch square pan. A 9x9-inch pan also works nicely. Or, bake it in a loaf pan or an 8 or 9-inch (lined) cake round for a more formal cake.
Fresh or frozen cranberries? Either fresh or frozen berries work fine. I like to buy cranberries in November and throw them in the freezer so that I can bake with them whenever I want. You don't need to defrost them first; just add them frozen to the batter.
To make dairy-free: Substitute the butter with non-dairy butter or coconut oil and substitute the milk with your favorite non-dairy milk.
Toppings. Try the lemon glaze, or dust the top of the cake with powdered sugar.
To Serve: Whipped cream, vanilla ice cream, or a drizzle of salted caramel sauce.