Preheat the oven to 350°F (175°C). Line a 9 × 5-inch loaf pan with parchment paper, leaving an overhang on the sides.
In a bowl, whisk together the flour, baking powder, baking soda, and salt.
In a separate large bowl, whisk the melted, slightly cooled butter with the granulated sugar. Add the eggs, buttermilk, and vanilla. Whisk until well combined.
Add the dry ingredients to the wet ingredients. Gently stir until no dry flour remains. Do not overmix.
Divide the batter into two equal portions. Stir the cranberry jam into one portion until evenly combined, leaving the other portion plain.
Spoon the vanilla and cranberry batters alternately into the prepared loaf pan until all the batter is used.
Loosely tent the pan with aluminum foil, leaving room for the cake to rise. Bake for approximately 1 hour, removing the foil during the final 10 minutes to allow the top to brown.
Insert a toothpick into the center. The cake is ready when it comes out clean or with a few moist crumbs, but no wet batter. If needed, bake longer, checking at 5-minute intervals. Cover loosely with foil again if the top is already browned.
Let the loaf cool in the pan for about 15 minutes. Carefully lift it out using the parchment paper and transfer to a wire rack. Cool completely.
For the glaze, beat the softened cream cheese until smooth. Mix in the cranberry jam, powdered sugar, milk, and vanilla until well combined.
Spoon the glaze over the cooled loaf, allowing it to drip down the sides. Cut into 10 slices and serve.