Preheat your oven to 350°F (175°C). Grease the 9x13 inch baking pan with butter or cooking spray to prevent sticking.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy using a whisk or an electric mixer.
Add the eggs one at a time to the butter and sugar mixture, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, combine the all-purpose flour, baking powder, baking soda, and salt. Gradually add this dry mixture to the wet ingredients, alternating with the milk. Mix until just combined.
Gently fold in the cranberries into the batter, being careful not to overmix; this will help distribute the cranberries evenly in the cake.
Pour the batter into the prepared baking pan and spread evenly.
Bake in the preheated oven for 40 minutes or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven and let it cool in the pan for about 10 minutes before transferring it to a wire rack to cool completely.