Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or line it with parchment paper.

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. Do not overmix.

Gently fold in the raisins or currants and lemon zest if using.

Pour the batter into the prepared cake pan and smooth the top with a spatula.

Bake in the preheated oven 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, then transfer it to a wire rack to cool completely.

Once cooled, dust the top with powdered sugar if desired before serving.