Preheat oven to 160C (325F).
Cake: Grease a deep 8 inch (20 cm) round cake pan. Line bottom with waxed paper and grease again. Pat maraschino cherries dry. In a bowl, combine flour, ground almonds, baking powder and salt. Add cherries and toss until pieces are well coated; reserve. In another bowl, beat butter, almond extract and sugar until light and fluffy. Add eggs one at a time, beating well after each addition. Stir in dry ingredients in three additions, alternately with sour cream. Spread into prepared pan. Bake 1-1/4 to 1-1/2 hours, or until toothpick inserted in centre comes out clean. Cool 10 minutes. Turn cake out and cool completely.
Custard: Heat milk and cream, without boiling. In a bowl, beat egg yolks with sugar; stir in hot milk and cream gradually. In a saucepan, cook over medium heat until thickened. Do not let mixture boil and stir constantly. Remove from heat. Stir in vanilla. Serve cold. To serve, spoon custard sauce over each serving of cake.