Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
In a large mixing bowl, cream together the butter and granulated sugar until light and fluffy. Beat in the egg and vanilla extract until well combined.
In a separate bowl, whisk together the flour, cocoa powder, baking powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a dough forms.
Turn the dough out onto a lightly floured surface and roll it out to about 1/4 inch thickness. Cut into rectangular shapes (about 2 inches by 1 inch) and place them on the prepared baking sheet.
Bake in the preheated oven for 10-12 minutes, or until the edges are set. Allow the cookies to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
In a mixing bowl, beat together the butter, powdered sugar, cocoa powder, and vanilla extract until smooth and fluffy.
Once the cookies are completely cool, spread a generous amount of the filling on the bottom side of one cookie and top with another cookie, pressing gently to sandwich them together. Repeat with the remaining cookies.
In a microwave-safe bowl, melt the chopped chocolate and vegetable oil in 30-second intervals, stirring between each, until smooth and fully melted.
Dip each sandwiched cookie into the melted chocolate, using a fork to turn and coat evenly. Allow any excess chocolate to drip off before placing the coated cookies on a wire rack or parchment paper to set.
Allow the chocolate to set completely before serving. Store the Tim Tams in an airtight container at room temperature.
Enjoy your meal!