Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, cream together the butter, sugar, and salt until light and fluffy.
Mix in the egg yolk and vanilla extract until well combined.
Gradually stir in the flour until a soft dough forms.
Roll the dough into 1-inch balls and place them on the prepared baking sheet.
Press a thumbprint into the center of each dough ball.
Bake for 8-10 minutes, or until the edges are lightly golden.
Remove from oven and gently press down the thumbprints again if needed.
Let cool completely before filling each center with melted chocolate.
Top each with a mint M&M or mint candy. Let set before serving.