Separate egg yolks and egg whites. In a large bowl, add the egg whites. Beat on high speed until soft peaks form, 2-3 minutes. Slowly add sugar, beat until stiff peaks form, 2-3 minutes.
In a separate bowl, with a mixer beat egg yolks and sour cream for 1 minute. Pour into egg whites bowl, beat on medium speed until egg yolks are incorporated.
In a bowl, combine flour, baking powder, and poppy seed. Add to the egg mixture. Mix for a few seconds on low speed until flour is combined. Turn off mixer and with a spatula, gently mix to make sure the flour is well incorporated. Don't overbeat or else the mixture will flatten. Pour mixture into baking sheet, spread evenly.
Bake 12 minutes or until toothpick comes out clean if inserted in the center of the cake. Slightly cool.
Run a knife along edges of cake to loosen from sheet. Remove cake roulade from baking sheet, cover with a kitchen towel and roll roulade up while it's still warm but not hot. Set aside to cool completely.
Prepare cream:
Beat butter on medium speed until creamy, about 3 minutes. Reserve 3 Tbsp of the dulce de leche for the topping, add the remaining dulce de leche to butter. Beat on medium speed until combined. Add powdered sugar, lightly beat again. Refrigerate cream until ready to use.
Assemble:
Unroll cake roulade. Generously spread cream, about 2/3 of the cream. Re-roll the cake. Cover top and sides of roulade with remaining cream.
Add toasted coconut to top of roulade and chocolate shavings. Add the 3 Tbsp remaining dulce de leche to the corner of a plastic bag, snip corner and drizzle over roulade.