Preheat and prepare: Preheat your oven to 180°C (160°C fan). Grease and line a 23cm round cake tin with baking parchment. The paper should come about 3cm above the rim — this helps the cake rise evenly without spilling.

Brown the butter for the frosting: In a small saucepan over medium heat, melt the 150g butter. Swirl the pan occasionally. You’ll hear it crackle and sizzle — that’s the water evaporating. After 4–5 minutes, the butter will turn a deep amber colour and smell intensely nutty. The moment you see golden-brown specks forming at the bottom, pour it into a heatproof bowl to stop the cooking. Let it cool to room temperature — about 20 minutes. Do not skip this step; hot butter will melt your icing sugar into a puddle.

Mix the dry ingredients: In a medium bowl, whisk together the flour, bicarbonate of soda, salt, and cinnamon. Set aside.

Cream the butter and sugars: In a large bowl, beat the 250g softened butter with the caster and brown sugars using an electric mixer on medium speed. Beat for 3–4 minutes until the mixture is pale, fluffy, and looks like thick sand. It should feel light when you lift the beater.

Add eggs and banana: Add the eggs one at a time, beating well after each. The mixture may look slightly curdled — that’s fine. Stir in the vanilla extract, then fold in the mashed bananas. The batter will look speckled and loose; this is the mark of a moist cake.

Alternate dry and wet: With the mixer on low, add one-third of the flour mixture, then half the buttermilk, then another third of flour, the remaining buttermilk, and finish with the last of the flour. Mix only until each addition disappears. Stop the moment the last streak of flour vanishes — overmixing will make the cake tough. The final batter should be thick, glossy, and drop reluctantly from a spoon.

Bake: Scrape the batter into the prepared tin and smooth the top. Bake for 45–50 minutes. At 40 minutes, check: the cake should be deep golden brown, and a skewer inserted into the centre should come out clean or with a few moist crumbs. If the top is browning too fast, loosely cover with foil for the last 10 minutes. Let the cake cool in the tin for 10 minutes, then turn out onto a wire rack to cool completely — at least 1 hour.

Make the frosting: Once the browned butter is cool but still liquid, beat it with the sifted icing sugar using an electric mixer. Start on low to avoid a sugar cloud, then increase to medium. Add the milk and vanilla, and beat for 2–3 minutes until the frosting is smooth, thick, and holds a soft peak. If it’s too stiff, add a teaspoon more milk; if too runny, add a tablespoon more icing sugar.

Assemble and present: Place the cooled cake on a serving plate. Using a palette knife, spread a thin layer of frosting over the top and sides to seal in crumbs (a crumb coat). Refrigerate for 15 minutes. Then apply the remaining frosting in swirls, waves, or a smooth finish. For a restaurant look, use a piping bag fitted with a large star nozzle to pipe rosettes around the top edge. Sprinkle flaky sea salt over the frosting just before serving.