Preheat your oven to 180°C (350°F) and prepare two 8-inch round cake pans.

In a heatproof bowl, combine the instant coffee granules with boiling water; stir until dissolved and cool to room temperature.

Sift together the flour, baking powder, and salt in a bowl and set aside.

Beat the softened butter and sugar together until light and fluffy, about 3-5 minutes.

Add the eggs one at a time, mixing well after each addition until combined.

Stir in the cooled coffee mixture and vanilla extract.

Gradually add the sifted dry ingredients, alternating with milk, mixing until just combined.

Fold in the chopped walnuts gently into the batter.

Divide the batter between the prepared pans, smooth the tops, and bake for 25-30 minutes until a toothpick comes out clean.

Let the cakes cool in the pans for 20 minutes before transferring them to a wire rack to cool completely.

For the frosting, beat the softened butter and gradually add powdered sugar, mixing until smooth.

Pour in brewed coffee and mix until creamy.

Spread frosting between the cooled cake layers and on top and sides of the cake.

Garnish with additional chopped walnuts before serving.