First, in a medium bowl, stir together the rolled oats and buttermilk. Let this sit for 15-20 minutes. This magical step makes the oats soft and gives the cake its wonderfully moist texture.
While the oats are soaking, preheat your oven to 350°F (175°C). Grease an 8x8 inch square baking pan or line it with parchment paper, leaving a little overhang on two sides to make lifting the cake out easier.
To the soaked oat mixture, add the brown sugar, melted butter, eggs, and vanilla extract. Whisk it all together until it’s just combined. It’s okay if it looks a little lumpy!
In a separate bowl, whisk the flour, baking powder, baking soda, cinnamon, and salt. Gently fold these dry ingredients into the wet mixture. Stir until you no longer see streaks of flour, but don’t overmix.
This is the fun part! Add most of your cranberries to the batter, saving a small handful for the top. Fold them in gently so they don’t get smashed.
Pour the batter into your prepared pan. Sprinkle the top with the reserved cranberries, the tablespoon of oats, and the coarse sugar. Bake for 35-40 minutes, or until the top is golden and a toothpick poked into the center comes out clean. Let it cool in the pan for at least 30 minutes before slicing.
