Step 1 Pre heat oven to 160ºc fan forced. Line 12 x 1/3 cup muffin tin with paper cases. Cut 4 Tim Tam biscuits into 3 pieces. Set aside.
Step 2 Use electric beaters to beat butter and sugar in a bowl until pale and creamy. Add eggs, 1 at a time, beating well after each addition. Fold in flour, cocoa and buttermilk until smooth.
Step 3 Divide mixture among cases. Place 1 Tim Tam piece (1/3 of a Tim Tam) into each cake batter, covering with batter.
Step 4 Bake for 20-25 minutes or until firm to touch. Stand cakes for 5 minutes then transfer to a wire rack to cool completely.