Prep the oven and pans
Preheat oven to 160°C (140°C fan-forced). Grease and line the bases of two 20cm round sandwich pans.

Whip the airy egg base
In a stand mixer, beat eggs on high ~5 minutes until pale and voluminous. Slowly stream in caster sugar; beat 3–4 minutes more to ribbon stage.

Dry & coffee mixtures
Sift flour, baking powder, and salt. Stir in chopped walnuts. Warm milk with butter until just melted; stir in coffee granules and vanilla until dissolved.

Fold the batter
Gently fold dry mixture into the whipped eggs until just combined. Fold in the warm coffee-milk mixture. Do not overmix.

Bake & cool
Divide batter between pans. Bake ~45 minutes until golden and a skewer comes out clean. Cool in pans 10 minutes, then turn out to a rack to cool completely.

Make coffee butter cream
Beat butter and vanilla until very pale and creamy. Mix in half the sifted icing sugar on low. Dissolve coffee in lukewarm milk. Alternate adding coffee-milk and remaining icing sugar; beat until smooth and fluffy.

Candy the walnuts
Toast walnuts in a dry skillet over medium-high heat until fragrant. Sift icing sugar over; stir as it melts and turns golden. Immediately tip onto baking paper and separate. Cool, then break into pieces.

Assemble
Level cakes if needed, then split each into two for four layers. Place first layer on a stand; spread ~⅕ of buttercream. Repeat with remaining layers. Crumb-coat sides lightly, then finish the top smoothly. Scatter candied walnuts just before serving.