In a small saucepan, heat milk over medium heat until barely simmering.
In a medium bowl, whisk egg yolks, sugar, and cornstarch until smooth. Whisking constantly, gradually add hot milk to egg mixture. Return milk mixture to saucepan, and cook over medium heat, whisking constantly, until mixture is thickened and coats the back of a spoon. Stir in amaretto. Spoon custard into bowl and cover with plastic wrap, pressing wrap directly onto surface of custard to prevent a skin from forming.
Refrigerate until very cold, at least 4 to 6 hours.
In a large bowl, beat butter with a mixer at medium speed until light and fluffy. Beating constantly, add custard in 3 additions, beating for 30 seconds between each. Continue beating until very light and fluffy, about 5 minutes.
Recipe by Victoria at https://victoriamag.com/plum-dacquoise-recipe/