Preheat your oven to 180°C (160°C fan) or 350°F. Grease and line a 9x9-inch baking tin with parchment paper.

In a large mixing bowl, combine the softened butter and light brown sugar. Cream together until pale and fluffy.

Add the eggs one at a time, beating well after each addition until smooth.

Sift in the self-raising flour and baking powder, then fold gently into the batter.

Dissolve the instant coffee granules in a small amount of hot water, then stir into the batter along with the chopped walnuts.

Pour the batter into the prepared baking tin and bake for 25-30 minutes, until golden brown.

Let the traybake cool in the tin for 10 minutes before transferring it to a wire rack to cool completely.

Beat the softened butter and icing sugar together until light and creamy, adding milk as needed.

Spread the icing evenly over the cooled traybake and decorate with walnut halves if desired.

Slice into squares or rectangles and serve with coffee or tea.