01 -
Set your oven to 350°F (175°C). Grab a 9×9 pan, spread on some butter and a dusting of flour, or just put down some parchment—whichever you like.
02 -
Start by blending flour, sugar, baking soda, baking powder, and salt with a whisk in a medium bowl. In a bigger bowl, cream the egg and butter till it gets a little fluffy. Pour in the coffee, buttermilk, and vanilla next—give it a good mix. Slowly fold the dry bowl into the wet bowl and stir until you don't see any more lumps.
03 -
Splash that batter into your prepped pan. Pop it in the oven for 25-30 minutes—poke with a toothpick and if it comes out dry, it's good. Let it rest in the pan for 10 minutes before sliding it to a rack to cool off all the way.
04 -
As your cake cools, grab a saucepan and toss in brown sugar, butter, and cream. Put it over medium heat and keep stirring till it starts to bubble. Let it keep going for 3 or 4 minutes so it thickens up, giving it a stir now and then. Pull it off the heat, mix in salt and vanilla, and let it wait till you're ready.