Line a 20 cm square tin with baking paper, leaving overhang on all sides to allow for easy removal.

Place the butter in a medium saucepan over low heat. Once melted, add the caster sugar and cook, stirring, for 3 to 5 minutes until the sugar has dissolved and the mixture is smooth.

Pour in the condensed milk and stir to combine. Add the cocoa powder, salt and espresso powder, stirring gently until fully incorporated.

Increase the heat slightly to low-medium and cook for 3 to 5 minutes, stirring constantly, until the mixture becomes thick, glossy and fudgy.

Remove from the heat and allow to cool for 2 to 3 minutes, until warm but not hot.

Fold through the finely crushed digestive biscuits until evenly coated, then gently fold through the biscuit chunks, keeping them intact for texture.

Transfer the mixture to the prepared tin and smooth the surface evenly. Refrigerate for 30 minutes to firm slightly.

Meanwhile, make the chocolate buttercream. Place the butter and icing sugar in a bowl and beat until pale, light and fluffy. Add the cocoa powder and espresso powder and beat until evenly combined. Add the vanilla extract and beat briefly to incorporate.

Spread the buttercream evenly over the chilled fudge base. Use the back of a fork to create a wave or striped pattern.

Return to the fridge for a further 30 minutes, or until fully set.

Lift the slice from the tin using the baking paper overhang. Using a sharp knife, cut into 9 squares, wiping the blade clean between each cut for neat edges.