Preheat oven to 350°F. Grease and line a 9″x9″ baking dish with parchment paper.
In a large bowl, whisk together the butter, brown sugar, and molasses until creamy, about 1 minute. Add in eggs, one at a time, beating between each addition. Add in vanilla extract.
In a separate bowl, sift together flour, baking powder, baking soda, salt, and spices. Use a spatula to fold into wet ingredients until incorporated. Fold in skyr. Batter will be thick. Place batter into prepared pan, smoothing batter to the edges.
To make the crumb topping, combine flour, brown sugar, spices, and salt. Pour in melted butter and toss with your hands to combine. Use your fingers to rub mixture together until it resembles the texture of wet sand and pieces the size of peas remain.
Crumble mixture on top of spice cake batter, then bake cake for 40 minutes. Allow to cool slightly before removing from pan and serving with toffee sauce.
For the toffee sauce
In a small saucepan over low heat, combine heavy cream, butter, sugar, and rum (optional). Heat for 5 minutes, until sugar dissolves completely and mixture begins to bubble.
Remove from heat and place into a cup with a pour spout. When ready to serve cake, pour toffee over the top. Cake is best served warm, day of!