Zest all oranges first, then juice them. Keep zest and juice separate.
Preheat oven to 180°C / 160°C fan / Gas 4. Grease and line two 20cm sandwich tins.
Place butter, sugar, eggs, flour, baking powder, orange zest, orange juice, and vanilla in a large bowl. Beat with an electric whisk for 2–3 minutes until smooth and pale.
Divide equally between tins. Bake for 22–25 minutes until golden and a skewer comes out clean.
Immediately mix orange juice and caster sugar for the drizzle. Poke holes over warm sponges with a skewer. Spoon drizzle over both. Leave in tins 15 minutes, then cool completely on a wire rack.
Beat butter alone for 3–4 minutes until very pale. Add icing sugar in two batches. Add orange zest, orange juice, and vanilla. Beat until smooth.
Sandwich sponges with half the buttercream. Spread remainder on top. Decorate with orange slices and a dusting of icing sugar.