Make the Cupcakes
Preheat oven to 350°F (175°C). Line muffin tins with paper liners (recipe makes ~30). In a large bowl, whisk together sugar, flour, cocoa, baking powder, baking soda, and salt. Add eggs, milk, oil, and vanilla; beat on medium speed for 2 minutes. Stir in the boiling water by hand (batter will be thin). Fill cups 2/3 full and bake for 18-20 minutes. Cool completely.

Cook Frosting Base
In a medium bowl, whisk sugar, flour, cornstarch, and salt. Slowly whisk in the milk until smooth. Crucial Step: Strain this mixture through a fine-mesh sieve into a saucepan to remove lumps. Cook over medium heat, whisking constantly, until it boils and thickens into a pudding-like consistency (5-10 minutes).

Cool Frosting Base
Transfer to a clean bowl. Cover with plastic wrap touching the surface and let it cool to room temperature. It must not be cold or warm.

Make Ganache
Place chopped chocolate in a bowl. Heat heavy cream to a simmer. Pour hot cream over chocolate and let sit for 2 minutes. Whisk until smooth, then whisk in the butter. Refrigerate, stirring every 10 minutes, until it reaches a thick, pipeable consistency (like toothpaste).

Whip Frosting
Once the flour base is room temp, beat it with vanilla on low for 30 seconds. Add the softened butter one piece at a time, beating until fully incorporated. Increase speed to medium-high and whip for 5 minutes until light and fluffy. Fold in the caramel sauce.

Assembly – Step 1 (The Ring)
Fit a piping bag with a large round tip (Wilton 1A). Pipe a ring of ganache around the top perimeter of each cupcake, leaving the center empty.

Assembly – Step 2 (The Crunch)
Pour toffee bits into a bowl. Gently invert the cupcake and press the ganache ring into the toffee so the bits stick to the chocolate.

Assembly – Step 3 (The Fill)
Fit a second piping bag with a star tip (Wilton 1M) and fill with the caramel frosting. Pipe a decorative swirl into the center of the ganache ring. Top with extra toffee bits if desired.