Coarsely chop the almonds and set aside.
Add eggs and sugar to a bowl and whisk thoroughly.
Heat gently in a double boiler while continuously stirring.
Sift cake flour and baking powder into the mixture and mix until well combined.
Add melted butter and milk, stirring until the batter is evenly mixed.
Add the chopped almonds and fold lightly.
Line a Castella pan with parchment paper and pour in the batter.
Bake in a preheated oven at 180°C for 30–35 minutes.
Remove to a cooling rack, let cool completely, then cut into serving pieces.