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Preheat oven to 350 degrees F
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Grease an 8 inch square baking pan with butter
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Line the pan with parchment paper, forming a divide in the middle with the parchment or aluminum foil.
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Whisk together caster sugar, flour, ground almonds and baking powder and combine with the butter and eggs in a large bowl. Beat together until the ingredients are combined and the batter is smooth
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Place half the mixture into a smaller bowl, add the vanilla, 11/2 teaspoons milk and chopped walnuts. Mix until smooth.
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Add the walnut batter into one side of the divided baking pan
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Dissolve the coffee in the remaining 11/2 teaspoon milk and add to the remaining batter. Mix until smooth.
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Add the coffee batter into the other half of the divided baking pan
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Smooth the surface of the batter with a spatula, making sure batter is in each corner
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Bake for 25-30mins until the cake is well risen, and a toothpick comes out clean.
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Leave to cool in the pan for a few minutes and then remove from the pan to cool completely.
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Once cool cut each section in half and continue to trim the edges until you have 4 even sized pieces.
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To make the buttercream: In a separate bowl combine the 1 stick butter, powdered sugar, 1/2 teaspoon instant coffee and 1 1/2 teaspoons milk. Beat until smooth.
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Spread a thin layer of buttercream on the inside strips of the cake to stick the strips together in a checkered pattern, one light next to one darker one and and switching the two underneath.
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Dust a large cutting board with some powdered sugar and roll the marzipan out into a thin rectangular shape - large enough to cover all 4 sides of the cake.
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Spread a layer of buttercream on top of the cake.
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Buttercream side down, place the cake on the marzipan.
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Continue to spread buttercream on remaining sides of the cake and press the marzipan around the sides.
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Flip the cake over so that the marzipan seam is on the bottom.
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Trim off excess marzipan and using a knife, score the top of the cake.