01 - Preheat oven to 350°F. Grease a 9-inch round cake pan and line bottom with parchment paper.
02 - In a small saucepan over medium heat, melt butter. Stir in brown sugar until dissolved and bubbling, about 2 minutes. Pour mixture into prepared pan and spread evenly.
03 - Scatter cranberries evenly over the sugar mixture in the pan.
04 - In a medium bowl, whisk together flour, baking powder, and salt.
05 - In a large bowl, beat softened butter with granulated sugar until light and fluffy, about 2 to 3 minutes.
06 - Add eggs one at a time, beating well after each addition. Mix in vanilla and orange zest if using.
07 - Add dry ingredients in three parts, alternating with milk, beginning and ending with flour. Mix gently until just combined.
08 - Carefully spoon batter over cranberries and smooth the top evenly.
09 - Bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean.
10 - Let cake cool in pan for 10 minutes. Run a knife around the edges and invert onto a serving plate. Remove parchment paper. Serve warm or at room temperature.