Beat the softened butter and powdered sugar for 2 to 3 minutes until light and fluffy. Mix in the egg yolk and vanilla extract until smooth.

Sift together the flour, cocoa powder, salt, and espresso powder in a separate bowl. Add the dry mixture to the butter mixture on low speed and mix just until a soft dough forms.

Split the dough in half, shape each piece into a rectangle, wrap in plastic, and refrigerate for at least 1 hour.

Preheat the oven to 350°F and line two baking sheets with parchment paper. Roll one portion of dough to about 1/4 inch thick, cut rectangles about 2.5 x 1 inches, and place on the sheets.

Bake for 10 to 12 minutes per tray until set but not hard. Let cool on baking sheets for 5 minutes, then transfer to a wire rack to cool completely.

Warm the cream and butter in a saucepan until just simmering, pour it over the chopped chocolates in a heatproof bowl, and let stand for 2 minutes.

Stir in the vanilla bean paste until the filling is smooth and glossy, then cool to a spreadable texture.

Spread filling on the flat side of one biscuit, top with a second biscuit, and press gently.

Melt the dark chocolate and coconut oil together in a heatproof bowl over simmering water, dip each sandwich cookie, let the excess chocolate drip off, and place on a parchment-lined tray. Let set at room temperature or chill until firm.