Heat the oven to 175°C/350°F conventional, or about 160°C/325°F fan-assisted. Grease two 8-inch/20-cm round cake pans and line the bottoms with parchment.

Whisk the flour, baking powder, and salt together in a medium bowl. Set this dry mixture aside while you prepare the batter.

Beat the softened butter and sugar for about 3 to 4 minutes, until pale and fluffy. Add the eggs one at a time, scraping down the bowl after each addition, then mix in the vanilla.

Add the flour mixture in two or three additions, alternating with the milk. Begin and end with the dry ingredients, mixing only until the batter is combined.

Divide the batter evenly between the prepared pans and smooth the tops. Bake for 25 to 30 minutes, until the cakes spring back lightly and a skewer comes out clean or with a few moist crumbs.

Cool the cakes in their pans for 10 minutes, then turn them onto a wire rack and cool completely. Do not frost warm layers, as the buttercream can melt and slide.

For the buttercream, beat the softened butter until smooth. Gradually add the powdered sugar, then add the vanilla, salt, and enough milk or cream to create a spreadable consistency.

Place one cooled cake layer on a serving plate, spread it with buttercream, and top with the second layer. Cover the top and sides with the remaining buttercream, then slice and serve.