In a bowl, whisk the boiling water into cocoa until smooth and whisk in milk and vanilla extract. In a large bowl, beat together butter and sugars until light and fluffy, about 2 to 3 minutes, and beat in eggs, 1 at a time, beating well after each addition. Into another bowl, sift together flour, baking soda, and salt and add to egg mixture in batches alternately with cocoa mixture, beginning and ending with flour mixture, and beating well after each addition. Stir in the dulce de leche. Fill each wrapper slightly more than 1/2 way. They will rise a lot. Bake in a preheated 350F oven for about 18 minutes. Cool on a wire rack.