01 -
Turn your oven on to 350°F (175°C). Grease your 9×9-inch (23×23 cm) baking pan with flour or just put some parchment paper in.
02 -
Grab a medium bowl and stir the flour, sugar, baking powder, baking soda and salt together. In another bigger bowl, beat your soft butter with the egg until they're fluffy. Pour in the cold coffee, buttermilk, and vanilla, then mix it up. Slowly add the dry stuff to the wet stuff and stir until it's smooth.
03 -
Put all your batter in the pan and stick it in the middle of your hot oven for about 25-30 minutes. You'll know it's done when you poke it with a toothpick and nothing sticks. Let it sit in the pan for 10 minutes, then move it to a wire rack to cool down all the way.
04 -
Put your brown sugar, butter and heavy cream in a pot over medium heat. Keep stirring until it starts bubbling. Let it bubble for 3-4 minutes while you stir it now and then, until it gets a bit thick. Take it off the heat and mix in the vanilla and salt. Let it cool a little.