Set the oven to 110 Degrees Celsius. Gas mark: ¼. Degrees Fahrenheit 225.
Line a small tin with butter. Chop all of the chocolate into small pieces. Break all of the Digestives in to small pieces. Place half of the broken chocolate in to the tin.
Then place all of the digestives on top. Then add the rest of the chocolate. Finally, thinly slice the butter on the top of the chocolate. Place in the oven for 50 minutes.
Once the chocolate has melted take it out of the oven and let it cool down. Once it is cool, place the tin in the fridge for two hours, until set really hard. Run a knife along the sides to help release it from the tin.