Combine chopped dried dates with bicarbonate of soda. Pour boiling water over the dates and set aside for about 20 minutes.

In a large mixing bowl, cream together softened unsalted butter and dark muscovado sugar for 3-5 minutes. Gradually add eggs, treacle, and date mixture. Fold in self-raising flour.

Preheat oven to 350°F (175°C). Prepare muffin tin with cupcake liners. Divide batter into liners and bake for 20-25 minutes.

Cool cupcakes in the tin for a few minutes then transfer to a wire rack.

Melt additional butter with more muscovado sugar in a saucepan. Stir in double cream and simmer for 5 minutes.

In another bowl, beat softened butter and sifted icing sugar until fluffy. Gradually add milk and vanilla extract to achieve desired consistency.

Frost cooled cupcakes with buttercream and drizzle warm toffee sauce over the tops.