Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a large mixing bowl, whisk together the flour, sugar, baking powder, baking soda, and salt until well combined.
Add the softened butter to the dry ingredients and mix until the mixture resembles coarse crumbs.
In a separate bowl, beat the eggs and then stir in the vanilla extract and buttermilk.
Pour the wet ingredients into the dry ingredients and mix until just combined. If using, fold in the raisins or currants.
Pour the batter into the prepared cake pan and smooth the top with a spatula.
Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.
Once cooled, dust the top with powdered sugar if desired before serving.