Preheat your oven to 375°F (190°C). Arrange the frozen tart shells on a baking sheet.

Fry the bacon in a skillet over medium heat until very crispy. Remove from pan, drain on paper towels, and crumble into small bits.

In a medium mixing bowl, whisk together the melted butter and packed brown sugar until smooth.

Add the maple syrup, beaten egg, vanilla extract, white vinegar, and salt to the bowl. Whisk until fully combined and the mixture is uniform.

Distribute the crumbled bacon evenly among the bottom of the 12 tart shells.

Pour the filling mixture over the bacon in each shell, filling them about 2/3 to 3/4 full. Do not overfill, as the filling will bubble up during baking.

Bake in the preheated oven for 15 to 20 minutes, or until the pastry is golden brown and the filling is bubbly.

Remove from the oven and let the tarts cool completely in the shells on a wire rack. The filling will firm up as it cools.