Beat egg whites with an electric mixer on medium speed until the soft peak stage. Gradually add sugar and continue beating on high speed until stiff peaks form. Fold in chopped cashew nuts and vanilla extract.
Transfer meringue into a piping bag and pipe the mixture into circles on lined baking sheets, about 2 ½ -3 inches in diameter and about ⅛-inch thick. You may use a cookie cutter as a guide, if desired.
Bake in a preheated 248°F oven for 15-20 minutes until wafer turns lightly brown. When done, let cool then remove the crisp wafers gently, being careful not to break them. Set aside some wafers to make crumbs.
To prepare the buttercream filling, combine sugar with water in a pot. Bring to a boil over low heat. Do not stir. Wait until big bubbles turn smaller and the liquid becomes syrupy or of thread-like consistency. Turn off heat.
Pour egg yolks into a bowl. Place the bowl on a double boiler and beat egg yolks until lemon-colored. Remove from heat.
Pour hot syrup into the beaten egg yolks and continue beating until thick. Let mixture cool down completely and then beat in the whipping cream.
In a separate bowl, cream butter using a mixer with a whisk attachment at medium speed until light and creamy. Gradually mix in cooled egg yolk mixture at low speed and beat until of spreadable consistency. Add the coffee mixture and mix well.