Walnut Cake with Mascarpone Cream

I made this for Easter, because I couldn't find all of the ingredients for tiramisu (where the heck does one find ladyfingers anyway???). The original recipe came from here. It's supposed to be a coffee cake, but I didn't have coffee, so I just made it without.

Also, it should be noted, as seen in the photo, that I should have let the cake cool completely before frosting it. There is supposed to be a middle layer of frosting, but it all got smooshed in and absorbed into the cake.

Cake

• 1 cup walnuts (about 4 ounces), toasted, chopped, divided
• 1 cup sugar, divided
• 1 tablespoon instant espresso powder
• 1 1/3 cups all purpose flour
• 2 teaspoons baking powder
• 1/2 teaspoon ground cinnamon
• 1/4 teaspoon baking soda
• 1/4 teaspoon salt
• 10 tablespoons (1 1/4 sticks) unsalted butter, room temperature
• 3 large eggs
• 1 large egg yolk
• 1/2 teaspoon vanilla extract
• 3/4 cup buttermilk

Coffee-mascarpone cream

• 4 1/2 teaspoons instant espresso powder
• 1 tablespoon water
• 1 cup chilled heavy whipping cream
• 1/2 cup (4 ounces) mascarpone cheese*
• 1/2 cup powdered sugar
• 1/2 teaspoon vanilla extract
•  3/4 cup (about 3 ounces) walnuts, toasted, chopped
• 1/4 teaspoon ground cinnamon
• Pinch of salt