Prepare the tin – Preheat the oven to 180°C (350°F) (160°C/325°F fan-forced). Grease and line a 20 cm (8 inch) square tin with baking (parchment) paper, leaving at least 5 cm (2 inches) overhang to help you lift the brownies out once baked.

Melt the butter – Melt the butter in a medium saucepan over low heat until just melted, or place the butter in a microwave-safe bowl and microwave in 30-second bursts until melted (about 60 seconds).

Mix the base – Transfer the melted butter to a large heatproof bowl. Add the brown sugar and stir until well combined and glossy. Set aside for 5 minutes to cool slightly. Add the eggs and vanilla and whisk gently until smooth and well combined.

Add the dry ingredients – Add the flour, cocoa powder and salt and mix until just combined – avoid overmixing.

Fold the chocolate through – Fold the chopped dark and milk chocolates through. Pour the batter into the prepared tin and spread evenly.

Heat the cream for the ganache – Heat the thickened cream in a medium saucepan over low heat until just hot, or microwave in a heatproof bowl in 15-second bursts (30–45 seconds total), until hot but not boiling.

Mix with the chocolate – Place the chopped dark chocolate into a medium heatproof bowl. Immediately pour the hot cream over the chocolate and stir with a spatula (not a whisk) until smooth and glossy. If the ganache hasn’t fully come together after 1–2 minutes, microwave in 10-second bursts, stirring well between each, until smooth.

Finish and set – Pour the ganache over the cooled brownie and gently tilt the tin to level. Sprinkle with heart sprinkles (if using). Refrigerate for 1–2 hours, or until the ganache is set but still soft to the touch.

Slice and serve – Lift the brownie from the tin, using the paper overhang to help you, and cut into nine squares using a sharp knife for jumbo brownies or 16 squares for smaller brownies, wiping the blade clean between slices.