Pre-heat oven to 350F.
Add the butter and chocolate chips to a saucepan set over medium heat. Cook, stirring frequently, until melted. Stir in the rum and let cool.
Put a couple of tablespoons of the chocolate mixture in a small bowl and stir in mustard until blended.
Add back into chocolate-rum mixture and beat in eggs one at a time.
Add flour, sugar, cinnamon, baking powder, baking soda, and salt to a mixing bowl. Stir well, breaking up any clumps of brown sugar that might have formed.
Stir in half the chocolate. Mix well. Stir in remaining chocolate and mix until combined.
Pour into a greased loaf pan.
Bake 45-55 minutes, or until the center of the cake is set and a knife inserted into the middle comes out clean. Remove from oven.
Allow cake to cool completely on a wire rack.
Invert onto a plate and remove the pan.
Add the pears and sugar to a sauté pan. Cook, stirring frequently for 5 minutes or until pears are slightly softened and golden. Spoon pears over cake.