Preheat oven to 350 degrees Fahrenheit. Line two sheet pans with Silpat mats or Gefen Parchment Paper.

In a mixer bowl, cream the butter and sugar until pale and creamy. Add egg and vanilla and beat until combined, scraping down the sides of bowl if needed.

Combine flour and salt. Gradually add to butter mixture and beat until smooth.

Using a medium cookie scoop (1 and 1/2 tablespoons), scoop dough and roll into a ball. Place cookie on prepared pan. Gently make an indentation in the center of each cookie using your thumb or the back of a melon baller. Refrigerate shaped cookie until firm, about 15 minutes.

Bake until edges are lightly golden, about 10 to 12 minutes. Press in the centers again if they are not so deep. Remove from oven and let cool completely.

Heat heavy cream in a small saucepan until bubbles form around the edges.

Place chopped chocolate in a small bowl and pour heated cream over. Let sit one minute to melt the chocolate. Stir gently to combine. Add the dulce de leche and stir until smooth.

Pour ganache into a disposable piping bag. Snip a small hole at the tip and fill each thumbprint cavity. Drizzle with white chocolate to garnish.