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 cups all-purpose 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
Batter will be thick. Your batter will be quite thick especially after you add the cranberries. Don't worry! This is totally fine. Use a rubber spatula to spread and smooth in in the pan.
Change up the pan. I like a thicker cake, so I baked this in an 8x8-inch square pan. A 9x9-inch pan also works great. Or, bake it in a loaf pan or an 8 or 9-inch (lined) cake round.
Add lemon zest. Whether or not you add the glaze, a bit of lemon zest rubbed into the sugar before you cream it with the butter will add a beautiful undertone of citrus.
Or try orange zest. Cranberry and orange is a classic pairing. Rub the zest of 1 orange into the 1 cup sugar (this releases its oil and makes the cake more flavorful), then proceed with the recipe.
Fresh or frozen cranberries? Either work beautifully. 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.