Preheat oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a medium bowl, whisk together flour, baking powder, and salt. Set aside.
In a large bowl, cream together butter and sugar until light and fluffy. Beat in eggs, one at a time, then mix in vanilla extract and buttermilk.
Add dry ingredients to wet ingredients and mix until just combined. Fold in poppy seeds.
Spread batter into prepared pan. Sprinkle brown sugar over top of batter. Bake in preheated oven for 25 to 30 minutes, or until a toothpick inserted into center of cake comes out clean.
Allow cake to cool in pan before serving.
Interesting Facts
This coffee cake is named for its coffee-like flavor, not because it’s meant to be served with coffee.
Poppy seeds are the seeds of the opium poppy plant, which has been used for centuries as a medicinal plant.
The buttermilk in the recipe adds a slight tang to the cake, while also helping to make it super moist.