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1
Melt chocolate and butter in a double boiler over a low simmer, stirring frequently until melted. Remove from heat and stir in Guinness, vanilla, and salt. Whisk in egg yolks one at a time, until thoroughly combined. Cool 5 minutes.
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2
Meanwhile, in an electric mixer fitted with a whisk attachment, whip egg whites on high speed into soft peaks. With mixer running, slowly add sugar, until thick and shiny. Slowly fold 1/3 of egg whites into chocolate mixture, then fold in remainder of egg whites until just incorporated, being careful not to overmix or mousse will lose volume.
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3
In an electric mixer fitted with whisk attachment, whip cream on high speed into stiff peaks. Slowly fold into chocolate mixture until completely incorporated. Put into serving bowls; cover and refrigerate for at least 2 hours for flavors to meld.
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4
, cook Guinness in a small saucepan over medium-high heat until reduced to 1/2 cup. Set aside. Preheat oven to 350 degrees.
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5
Place butter and both sugars in an electric mixer fitted with a paddle attachment. On medium-high speed, cream until light and fluffy, 2-3 minutes. Add eggs, reduced Guinness, and vanilla, beat until combined, scraping down the bowl as needed. Add oats, both flours, baking powder, baking soda, and salt until just combined, 15-30 seconds. Stir in chocolate chunks.
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6
Scoop 1/2 tablespoon portions of cookie dough onto parchment lined baking sheet, leaving a 1 inch space between each. Bake 6-9 minutes, cookies should look slightly underdone when you remove them from the oven. Let cool on cookie sheet 5 minutes, transfer to wire rack to cool completely. Keep raw cookie dough in refrigerator while baking.
