Preheat Oven: Set oven to 175°C (350°F). Grease and line an 8-inch round cake pan.
Toast Walnuts: In a dry pan, toast walnuts until golden. Set aside to cool.
Mix Dry Ingredients: In a bowl, whisk together flour, sugar, baking powder, baking soda, and salt.
Mix Wet Ingredients: In another bowl, combine oil, milk, vanilla, and dissolved coffee.
Combine: Pour wet ingredients into dry ingredients and gently fold until combined. Add toasted walnuts. Do not overmix.
Bake: Pour batter into pan. Bake 30–35 minutes or until a toothpick comes out clean. Cool completely.
Prepare Buttercream: Beat butter until creamy. Gradually add powdered sugar, coffee, and vanilla. Add milk if needed for spreadable consistency.
Slice Cake: Slice cake horizontally into 2–3 layers.
Assemble Torte: Spread coffee buttercream between layers. Frost top and sides. Sprinkle extra toasted walnuts on top.
Serve: Chill 30 minutes for best results. Slice and enjoy with coffee or tea.