In a large mixing bowl, cream unsalted butter with brown sugar and granulated sugar until light and fluffy, about 3-4 minutes. Add the egg yolk and vanilla extract, mixing well until fully combined. Gradually incorporate all-purpose flour and a pinch of salt, stirring just until the dough comes together; fold in toffee bits and optional chocolate shavings.

Divide the dough into two equal portions and shape each into a log approximately 2 inches in diameter. Wrap tightly in plastic wrap and chill for at least 2 hours, or overnight.

Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

Slice the chilled dough logs into 1/4-inch rounds and place on the prepared baking sheet, leaving space between them.

Bake for 10 to 12 minutes, or until the edges are lightly golden. Remove from the oven and allow to cool on the sheet for a few minutes before transferring to a wire rack.

Consider adding a drizzle of caramel sauce on cooled cookies for extra sweetness.