Preheat your oven to 350°F (175°C). Grease and flour two 9-inch round cake pans.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt.
Add the softened butter, eggs, key lime juice, key lime zest, vanilla extract, and buttermilk to the dry ingredients. Beat on medium speed for about 2-3 minutes until the batter is smooth and well combined.
Divide the batter evenly between the prepared cake pans. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
While the cakes are cooling, prepare the raspberry glaze. In a small saucepan over medium heat, combine the fresh raspberries and key lime juice. Cook for about 5 minutes, mashing the raspberries with a fork until they break down.
Remove from heat and strain the mixture through a fine mesh sieve to remove the seeds. Return the liquid to the saucepan and whisk in the powdered sugar until smooth. Cook for an additional 2-3 minutes until slightly thickened. Let cool.
Once the cakes are completely cooled, place one layer on a serving plate. Spread a layer of raspberry glaze on top, then place the second cake layer on top. Drizzle the remaining raspberry glaze over the top of the cake, allowing it to drip down the sides.
Serve immediately or refrigerate for up to 2 hours before serving for a chilled treat.