Preheat oven to 325°F. Spray mini muffin tins with nonstick cooking spray.: The room fills with a warm, faintly sweet smell as the oven begins to climb, creating a steady backdrop for the rest of the work. Preheating ensures the shells start to set immediately, which helps them hold shape rather than slumping, and spraying the tins prevents sticking so the delicate shells come out clean and intact. A common mistake is not fully preheating, which can cause the shells to bake unevenly and collapse. Check the oven thermometer if yours runs hot or cold to keep the actual temperature accurate.
Beat butter, shortening, and cream cheese with an electric mixer on medium to high speed for 30 seconds. Add brown sugar, baking powder, and salt. Beat until combined, scraping the sides of the bowl occasionally. Beat in the egg and vanilla until combined. Slowly mix in the flour.: When you cream these fats, you'll notice the mixture lighten and develop a slightly whipped texture, which helps create a tender shell. The sugar melts into the fats, releasing a warm caramel scent as it blends. Scraping the bowl ensures even mixing so no pockets of unmixed fat remain, which would create uneven texture in the finished crust. If you rush the flour stage and overmix, gluten will develop, making the crust tougher, so stop as soon as the dough comes together. If the mixture feels too soft, chilling briefly helps firm it up for shaping.
If dough is too sticky to handle, you can chill it for 30-60 minutes. Otherwise, continue below.: Chilling tightens the fats and firms the dough, making it easier to roll into even balls and press into cups, and you will notice the dough becomes less tacky and more manageable. This resting also relaxes any gluten that formed, yielding a more tender crust after baking. A pitfall is leaving the dough out too long after chilling so it softens excessively; work quickly or chill in batches to maintain shape. If the kitchen is very warm, a longer chill is often necessary to avoid sticky hands and uneven shells.
Shape dough into 36 balls (about 1 1/4 inches each). Lightly press balls onto the bottoms and up the sides of the muffin cups.: As you shape each ball, feel for uniform density, because evenly sized shells bake at the same rate and produce consistent texture across the batch. Lightly pressing forms a thin base with slightly higher walls, which will contain the filling without becoming dough heavy. Pressing too hard can compress the dough and yield a dense crust, while pressing too lightly risks gaps and thin spots that collapse when filled. Keep a gentle, steady pressure as you build the shell walls.
Make the filling: stir chocolate and butter in a small saucepan over low heat until melted. Remove from heat. Stir in egg, brown sugar, and vanilla and peppermint extracts. Spoon 1 slightly rounded teaspoon of filling into each dough-lined muffin cup.: stir chocolate and butter in a small saucepan over low heat until melted. Remove from heat. Stir in egg, brown sugar, and vanilla and peppermint extracts. Spoon 1 slightly rounded teaspoon of filling into each dough-lined muffin cup. : Low heat prevents scorching the chocolate and keeps the mixture glossy as the butter melts into it, releasing a rich cocoa aroma. Once removed from heat, the residual warmth helps integrate the egg without scrambling it, and the sugar dissolves into a silky base, carrying the mint and vanilla notes. Spoon carefully so the filling seats within the shell without overflowing, aiming for uniform amounts so each tart bakes consistently. A typical blunder is adding the egg while the chocolate is too hot, which can curdle the filling; always allow a brief cool moment off the heat before stirring the egg in.
Bake for 15-20 minutes until the crust is just firm and filling is puffed and set. Cool in muffin cups on a wire rack for 5 minutes. Carefully remove tarts from muffin cups; cool completely on a rack.: During baking the kitchen fills with a toasty, chocolaty scent, and you will watch the filling puff slightly and develop a glossy surface. The shells will turn a pale golden hue around the edges, which signals they are done. Cooling briefly in the pan stabilizes the structure so they release cleanly, and finishing on a rack prevents sogginess by allowing air circulation. A common error is overbaking, which dries the filling and makes the shells brittle, so check frequently toward the end of the time window and remove when set but still slightly soft at the center for the best texture.
While the tarts are cooling, make the chocolate ganache. Place the chocolate in a medium sized bowl. In a small saucepan, heat the whipping cream over medium heat until it boils. Pour the cream over the chocolate and let sit 5 minutes. Stir until smooth.: The hot cream blooming the chocolate releases a deep, warm chocolate aroma and yields a satiny ganache that is glossy and pourable. Letting the cream sit before stirring gives the heat time to soften the chocolate gently, ensuring a smooth emulsion. Stirring slowly produces a mirror like sheen, and the finished ganache should move like liquid silk. If the ganache looks grainy, keep stirring gently or warm it briefly over a water bath to reemulsify; avoid overheating which can separate fats and ruin the texture.
Spoon a little ganache over each cooled tart. Sprinkle with crushed candy canes for garnish. Store in an airtight container for up to 3 days or freeze in a single layer in an airtight container or bag for up to 1 month.: The cooled tarts accept the warm ganache with a satisfying glossy finish, and the contrast of the cold baked shell with the still slightly warm ganache creates an inviting temperature play. The crushed candy canes add visual sparkle and a crisp minty crack as you bite in, completing the holiday profile. For storage, airtight containers preserve texture and prevent the candy from softening, and freezing in a single layer avoids sticking together. A mistake to watch for is stacking warm tarts, which traps moisture and makes the shells soggy, so ensure they are fully cooled before packing.