Preheat your oven to 350°F (175°C). In a mixing bowl, whisk together flour, baking powder, and salt.

In a separate bowl, cream unsalted butter with granulated and brown sugar until fluffy. Beat in the eggs one at a time, then add buttermilk and vanilla extract.

Gradually add dry ingredients to the wet mixture, folding in cranberries gently.

Grease a 9x9-inch pan, pour the batter, and bake for 30 to 40 minutes until a toothpick inserted comes out clean.

For the warm butter sauce, melt butter in a saucepan, add sugar and heavy cream, and whisk until smooth and bubbling. Stir in vanilla and let simmer for 2-3 minutes.

Let the cake cool for about 10 minutes, slice, and serve warm with the butter sauce.